Erm...have you been to LA?
I was actually just thinking that LA would be another exception. By the rest of the US I meant stereotypical "middle America".
There are probably fashiony-skinny areas of the UK outside of London as well.
Basically, you are going to find more skinny women in areas where fashion and style are a big part of the environment.
Regarding UK men being less superficial - sorry, but I have heard UK men make obnoxious comments about women who aren't as thin as they would like (despite not being skinny chickens themselves), probably no more, no less than I have heard in the US. Maybe British men are just too polite/cowardly to say it directly to the woman's face. Fortunately, not all men are like that.