I think clothes shopping for larger sizes is easier in the US - more stores that carry plus sizes, more selection, etc.
I find more size acceptance in the UK than I ever found in the US - that it's okay to be curvy, etc. I mean - look at Nigella, who is not fat, btw - but she'd never get on TV in the US, she'd be called fat over there. But I don't live in London (it's nice to visit, but...) so maybe it's different there.
As far as the whole 'fat Americans' thing, I don't encounter that much here and there are just as many 'fat Britons' as far as I'm concerned, if anyone wanted to get into that with me (but I don't encounter it, so no probs). Personally, I feel much more relaxed & more accepted, being bigger here, than I did in the US - now if I could just find clothes that I like & that fit!
(And yes, the sizing tends to be different - you will usually wear a bigger size in number here than you did in the US. In the US, I can usually wear around women's size 18, give or take, depending on the garment. Here, depending on the garment, it's usually a size 20-22. But it's just a number - who cares?)