"It's incredibly satisfying, it's been an incredible four weeks," McCullum told Test Match Special.

"We will walk away from this tour knowing we have achieved something very special and we will never forget it."

Prior to this tour, England had not beaten Pakistan in a Test outside of the UK for 22 years and had only ever won two away matches in 30 attempts against the same opposition.

But in their first Test series in Pakistan since 2005, England recorded only their fourth ever 3-0 away clean sweep against any team.

In the first Test in Rawalpindi, England piled up 506-4 on the first day, but only secured victory on an incredibly flat pitch in fading light at the end of the fifth day.

A tense finish in the second Test in Multan resulted in England prevailing by 26 runs, then the emphatic win in the finale in Karachi came after suffering the hindrance of losing the toss.

All this came against the backdrop of an illness that has lingered around the squad since the first day of the first Test was at risk of being delayed by 24 hours.

"Every ground has challenged us in different ways," said New Zealander McCullum. "Some challenged us tactically, some challenged us technically and some challenged us mentally.

"Then you add in the guys were really ill during this tour. To think they have bee able to play the style of cricket in those conditions we have on those three different grounds, we got pretty close to perfect."

The tour has seen the emergence of batter Harry Brook, who scored hundreds in each of the three Tests.

Brook got his chance after Jonny Bairstow suffered a broken leg, but McCullum said Bairstow will come back into the team "as soon as he is fit".

England also made 18-year-old Rehan Ahmed their youngest man to play international cricket in the final Test, with the leg-spinner taking 5-48 in the second innings.

McCullum believes Ahmed could continue his development in franchise cricket - the Leicestershire man is entered in Friday's Indian Premier League auction.

"The important thing now is that we continue to look after him," said McCullum. "I'll encourage him to play as much franchise cricket and get as many experiences around the globe and play with different players, under different coaches and in different conditions.

"It would be great if he got picked up at the IPL. Why not give him that opportunity to rub shoulders with other players and pick up those experiences? What other 18-year-old is going to get those chances? He's a fully fledged international cricketer."

England have now won nine of their 10 Tests since McCullum and captain Ben Stokes took charge at the beginning of the home summer.

"We all understand what we have done is pretty special," said Stokes. "To win 3-0 out here will be something to look back on and be very proud of.

"It's a great time to be playing for England. It's obviously a lot easier when we are winning - the real test will be if things don't go so well. I hope we don't come to that."

England's next Test assignment is two matches in New Zealand in February. They play a one-off match against Ireland in June before they begin the bid to regain the Ashes from Australia.

"I don't like looking too far ahead," Stokes told Sky Sports. "I obviously have my eye on the Ashes and have got little things about that in the back of my head.

"We will just continue to grow as a team and keep having fun, playing cricket with a smile on our face and win as much as we can."