On a warm morning earlier this month, a group of Metropolitan Police diplomatic protection officers sat in an anteroom off the ornate entrance hall in London's Lancaster House, sipping tea and nibbling chocolate biscuits, while upstairs a core group of European politicians discussed the future of European cooperation.

It was an apt setting: everywhere you look in Lancaster House, there is evidence of the long, entangled histories of the UK and Europe. The double sweep of its grand staircase deliberately echoes the Palace of Versailles. Queen Victoria sat in these rooms listening to Frederic Chopin play the piano in 1848. Tony Blair hosted Russian President Putin here for an energy summit in 2003.

The important issues on the agenda at the Lancaster House meeting, which was hosted by the Foreign Secretary David Lammy, included the latest developments in the war in Ukraine, Europe's response to ensure the continent's security, and – for the first time since Brexit – a summit between the UK and the European Union, which will take place on 19 May.

The British government believes it's a significant moment.

Before Brexit, British prime ministers would travel to Brussels four times a year or more for summits with the heads of the EU's institutions and its 27 member states. The haggling would go on late into the night. After Brexit those large summits stopped.

Now, the Labour government, elected last year on a manifesto that promised "an improved and ambitious relationship with our European partners", envisages new and regular interactions with the EU. Monday's marks the first.

Sir Keir Starmer will host the most senior EU leaders to launch a new "partnership".

Pedro Serrano, the EU ambassador to London, has described it as the "culmination of enhanced contacts at the highest levels since the July 2024 [UK] elections". But what will it amount to?

Is what's coming a "surrender summit" as the Conservatives warn; "the great British sellout" undoing bits of Brexit that Reform UK fear; or "a huge opportunity" the UK may be about to squander, as Liberal Democrats say? Or could it be an example of how, in Sir Keir Starmer's words, "serious pragmatism defeats performative politics" by delivering practical things that will improve people's lives?

In those long, drama-filled nights of 2020, when the then-prime minister Boris Johnson was negotiating Brexit, the possibility of a Security and Defence Partnership was discussed. But the UK's main priority was diverging from Brussels. So nothing was agreed – a notable omission, some think.

Now a new UK-EU security pact has been worked on for months, the plan is for it to be the centrepiece of what's agreed.

Kaja Kallas, the EU's foreign policy chief, who is overseeing negotiations, was at the early talks at Lancaster House. "Our relationship has had some difficulties," she told me, but "considering what is going on in the world […] we need to move forward with this partnership."

Yet some think the UK should not seize this outstretched hand.

"The cornerstone of our defence is Nato," Alex Burghart, a Conservative frontbencher, told the Commons this week. "We know of no reason why Nato is insufficient."

Reform UK's deputy leader Richard Tice has his own view. "There's no value at all," he argues. "We do not want to be constrained by a bungling top-down bureaucratic military structure. Our defence is guaranteed by Nato."

The government fires back on that point, arguing that a partnership will in no way undermine Nato; rather it will complement it, they say, because it will stretch to areas beyond defence, like the security of our economies, infrastructure, energy supplies, even migration and transnational crime.

Some industry experts also believe that a security pact could boost the UK economy. Kevin Craven, chief executive of ADS Group, a UK trade association that represents aerospace, defence and security firms, is among them.

Take, for example, the SAFE (Security Action For Europe) programme that is being set up by the EU, aiming to provide up to €150bn (£126bn) in loans for new projects. If the UK strikes a security partnership with the EU, then British weapons manufacturers could potentially access some of that cash.

"There is a huge amount of interest from European partners," says Mr Craven. "One of the challenges for defence companies in the last couple of years, since the advent of Ukraine, is being able to scale up their own capacity to meet demand." He estimates the UK could boost the EU's defence output by a fifth.

The Liberal Democrat's Foreign Affairs spokesperson, Calum Miller, similarly believes that a security pact is a huge opportunity for the British defence industry - but, he adds, "as importantly, it's a new strategic opportunity for the UK to be part of that ongoing conversation about how we arm as a continent".

Others point out that the UK has already been working with the EU on defence ever since Russia's invasion of Ukraine – at Nato, and most recently via the so-called Coalition of the Willing.

So, in practice, does it make huge amounts of difference to the UK's place in Europe?

No, argues Jill Rutter, a former senior civil servant who is now a senior fellow at the UK in a Changing Europe think tank. "Because relations [on defence] have already been improving quite a long way."

Some of those working on the partnership, however, argue that it will set in train new ways for the UK to engage and cooperate with its neighbours.

More contentious is the UK's desire to sign what's called a 'veterinary' deal to remove some border checks on food and drink. Nick Thomas-Symonds, the Cabinet Office minister leading these negotiations, told the Commons this week that the objective to lower food and drink costs is in the manifesto, so there is a mandate for it.

Inside the food industry, calls for reform have been growing. Julianne Ponan, whose firm Creative Nature makes vegan snack bars, exports to 18 countries but only a small proportion goes to the EU. She says this is because of the paperwork and inspections since Brexit.

One of her employees had to carry samples in her luggage on a passenger flight to Spain for a meeting to make sure the food wasn't held up at the border, she says.

"I think this will open up huge opportunities for businesses like mine."

It would also make a big difference for Northern Ireland. After Brexit, special arrangements were put in place to keep the land border it shares with the Republic of Ireland free of new impediments. But at the same time, new paperwork and inspections were put in place on food, drink, animals and plants being transported from mainland Britain across the Irish Sea to Northern Ireland. They're bitterly opposed by Unionist politicians and complicate life for many businesses.

But a veterinary deal may carry political danger. It would require the UK to align some of its rules on food and drink with EU ones, and move in-step with Brussels over time. And those rules are subject to oversight by EU courts.

"I call it the surrender summit," says Andrew Griffith, the Conservative Shadow Business and Trade Secretary. Under this deal the UK would lose "our freedom to set our own rules", he adds.

The Conservatives say they "fought long and hard" to "take back control of our laws, our borders, our money" – and that this should not now be reversed.

Reform UK has not held back in its language: "We think prepare for the Great British sell out. That's the bottom line, and it will be dressed up as a reset," Richard Tice says.

"Why would you want to reset and get closer to a patently failing economic model? The EU is struggling even more than we are. We should be diverging as fast as we can away from that."

But Labour's Thomas-Symonds dismisses these views as a "rehash of the arguments of the past".

On the other end of the spectrum is the accusation that Sir Keir is far too cautious. Calum Miller of the Liberal Democrats says he knows of businesses "gnashing their teeth in frustration that they just can't exploit opportunities to work with and trade with Europe".

His party wants the UK to explore a Customs Union with the EU. It would make moving goods easier, but mean we couldn't sign our own trade deals.

David Henig, a former senior trade negotiator, has been talking to both sides "hoping to help, to sort of navigate them in".

"The summit is a step forward, not a step change," he says, "A slight deepening of the trade ties, rather than something dramatically new."

A deal on food and drink checks would deliver very little, he believes, because food and drink is such a limited part of trade. "If you were, for example, aligning UK and EU rules on industrial products, you'd get a much bigger economic impact".

Jill Rutter thinks that a veterinary deal would not prove "economically earth shattering" – but if it goes well, she argues that it could provide "early proof of concept" for further UK-EU cooperation.

After Brexit, many British fishermen were disappointed when Boris Johnson's government agreed to let EU boats continue much as before, taking significant catches from UK waters. Those arrangements expire next year. The EU wants them extended.

David Davis who, as Brexit minister, led some of the original negotiations for the UK, told me fishing was "totemic" for Brussels. London conceded too easily, he thinks.

"Europeans got what they wanted first, and then we had a haggle from a weak position."

So he adds, "If I was giving advice to the government, I would say, tough it out" and use fishing as a lever to seek concessions.

But, as the UK found before, Brussels has cards to play. Much of the fish caught by British fishermen is sold to buyers on the Continent and the UK needs access to that market.

Some EU coastal states, like France and Denmark, are prepared to drive a hard bargain, demanding that London concedes on fishing rights in return for things it wants. Early on, even signing the Security Partnership was being linked to agreement on a fishing deal. The haggling will be tough.

And finally, there's an idea that has prompted much interest in recent months: a youth mobility deal, through which under-30s from the UK and EU could live and work in each other's countries.

For a long time the government said there were "no plans" for such a deal – but earlier this month they changed course, with Labour's Thomas-Symonds saying that "A smart, controlled youth mobility scheme would of course have benefits for our young people".

It's likely that would mean very limited numbers allowed to enter the UK, and only with a visa, for a limited time.

Under those conditions, ministers hope it would not inflate net migration numbers. It's far from what the EU would like.

The UK already has similar schemes with 13 countries, including Australia, New Zealand and Japan.

"When we are comfortable having those relationships, why are we so averse to having it with our nearest neighbours?" Calum Miller asks, "It just doesn't really make sense".

Paula Surridge, a professor of political sociology at Bristol University, argues that public views on immigration are more nuanced than many people think. "Voters care most about what they perceive as illegal migration – small boat crossings and so on," she says, "People coming here to study or to work, particularly young people, are not a particular cause for concern" for most.

"There will definitely be a group of voters that are upset [about potential deals], but they were never going to vote Labour."

Of those who backed Labour in 2024, she adds, about three quarters previously voted Remain in the Brexit referendum. The political risk to the government of signing pacts with the EU is "smaller than it appears", she adds.

Conservative pollster Lord Hayward is more cautious – and is concerned that a deal may pose a "bear trap" for the government if it's seen as providing free movement to young Europeans. "It will provide serious difficulties for them to come to an agreement on something which could easily be portrayed as EU membership 2.0."

Even before Sir Keir's upcoming summit on Monday, his opponents are raising that spectre.

"All of his muscle memory has been to get closer to the European political union," says Mr Griffith. "I am worried about our prime minister, with that baggage, with those preconceived ideas, […] trying to negotiate a better deal with the EU."

Richard Tice says his party could simply undo any deals with the EU. "If I'm right about our fears, and we win the next general election, we will just reverse the lot. The whole lot."

But Mr Thomas-Symonds is of the view that Monday will show the government is "not returning to the Customs Union, Single Market, or Freedom of Movement", all red lines it has pledged not to cross.

Instead it will be about "making Brexit work in the interests of the British people".

Back at Lancaster House, the politicians have moved on, heading to more meetings in Albania and Turkey to grapple with the issues facing the continent. But in a quiet hallway in the house is a painting from the 1850s of the Duke of Wellington inspecting troops in London's Hyde Park.

In it, he sits on a black stallion, raising his white-feathered hat to salute the cavalry - a tribute to the prime minister and military hero who defeated Napoleon at the Battle of Waterloo.

The upcoming summit won't be as momentous an event in the UK's complicated history with Europe. But a modern British leader about to plunge into the fray of European politics might pause for thought here – perhaps, for just a moment.

BBC InDepth is the home on the website and app for the best analysis, with fresh perspectives that challenge assumptions and deep reporting on the biggest issues of the day. And we showcase thought-provoking content from across BBC Sounds and iPlayer too. You can send us your feedback on the InDepth section by clicking on the button below.