At first I thought, that would be a great idea, but then I put some real thought into in and came up with these answers,
Reasons it would work:
First off to those who say no because there are no Vaults in England, May I remind you of the Ghoul community, or New Vegas that survived with anti missle Software, And think back to Little Lamplight (Fallout 3) Where the students who were on a ''Field trip'' to the caves when the bombs fell, they survived in the deep caves, then they started eating their teachers but that doesn't really matter.
There are plenty of caves (in Cheddar and many other places) in England that people could of survived in due to sheer luck or the delay from the bombs landing in America and people presuming England would also be hit, And there are a few people with bomb shelters etc that would be exposed to radiation once they left, but Ghouls gotta come from somewhere,
Reasons it wouldn't work:
The wildlife, in America, there are Coyotes and Scorpions and bears that can savagely kill you (In game...) In England... You're gonna get pwned by a evil Cow(Brahmin)? Sheep(Bighorner)? Squirrel?
And think about it, only in America would people start shooting each other and becoming bandits, Warring tribes, Warring Factions, Or nut jobs that think they're Caesar come to power after being bombed the shit out of by China instead of rebuilding society... And it takes a certain amount of inbreeding before radiation will turn you into Supermutants, and lets face it, only American has enough to have a significant amount of Mutants without them being wiped out within a week or two...