I think it is because you guys adopted orthodontics later than the USA.
I might be wrong on the dates but I think orthodontics was real common in the USA around the 1970's and you guys didn't really start doing it until the 1980's.
I think it is about crooked teeth and not bad oral hygiene.
So I think there is a generational thing where if you compare someone in their 30's to 40's will have straighter teeth in USA than in UK. Those younger than that probably are no different.
This is all a guess, though.
Paul Potts is an example, but not sure of his age.
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Edit: Well you might be right (1971), but then again it depends on how quickly society adopted it.
Take a look at this page... I don't know if it is true or not but here is what it says:
"Not nearly as many Europeans have braces as in the United States. They don't consider it as important, and that they are "just for looks."