british diet or bad dentistry...
I honestly think it could be both. The British do have a very sweet tooth and love their fizzy, sugary drinks. Plus with NHS dentists, the onus was on making sure the teeth were fit for eating, not beautiful. So I think it's a bit of both really. Dental health is vastly improving in the UK though.
Another thing was importance of the dentist in the UK. I heard numerous tales about it being too much hassle to get little timmy or jemima to go to the dentist because they hated it and so parents wouldn't bother making them go. My cousin is a classic example and her teeth are a disgrace. I personally like my smile, few wee chips that I should get fixed but I never have the time to fix and there is still that little gap, but it's hardly noticable.