During World War II, the LNER was experiencing a locomotive shortage, specifically locomotives for heavy goods, such as tanks and other military vehicles, but also to beat out their competitors. They decided to look across the pond for inspiration, and heard of the success of the Challenger's success and decided to purchase their own, with special permission from Parliament. It was classified as a Y1 class. It proved surprisingly successful, and with some modifications, was a hit with crews. Crews and Railfans nicknamed the locomotive "The Jolly Giant", due to how it seemed to take any job happily. Rarely struggling. The only mishaps that ever happened were occasionally couplings would snap due to the immense power of the locomotive. There was a bit of a problem with finding replacement parts, as they had to be imported. After the war was over, the Jolly Giant was put mostly aside, only really being brought out for special jobs. It's still somehow managed to last until the end of steam, where it managed to compete with its diesel replacements for quite a while, being one of the last steam engines taken off British Railways' roster. It ended up being preserved, and still runs excursions on the main line occasionally, running its own version of 4014's USA tour across the UK. For a special event, the Jolly Giant was taken back to America for a reunion event featuring it, UP 3985, and UP 4014, with them triple heading a train together. 

If this gets popular I might expand this!

The LNER Challenger, "The Jolly Giant" (very stupid alternate history idea that ignores loading gauge.)