Actually when I was travelling by train across the US from San Fransisco to Miami, & use to stop off for a week or 2 here or there at different places (like New Orleans & Birmingham, Alabama) I saw a lot more people with dud teeth than I ever saw in the UK.
I think its more an image thing - most Americans see themselves through the TV & the picture they get of themselves through the media (its probably true to a certain extent everywhere), & in TV land & Hollywood, etc everone has perfectly bleached white big flash teeth (that don't exist in nature). Whereas British films & TV programs are more reality based (even if they are comedies or science fiction or something), in that a lot more of the charactors have a average appearance, or are fat or distinctive & have average sort of teeth - its strange really, you get a lot more fat charactors on Brtitish TV than American TV, but in reality Americans are on average visably fatter than Brits.
Sure amongst people over 50, Brits are less likely to have perfect teeth than Amercans the same age, because back when they were growing up most of their parents couldn't afford more than the most rudimentary of family dental care & saw dentists a lot less than Americans of the same age group.
But since national health came in & later started to cover quite a bit of dental care, Brits under about 40 have teeth on average as good or better than average peoples anywhere else. I'd even go to say that the average working class Brit under 40 has better teeth than the average working class American, & the same goes for the lower middle class too. But at the opposite extreme, the wealthy Brits & British celibrities are probably on average a lot less image conscious than wealthy Americans or American celibrities, consequently they dont have that bleached big white straightened teeth look, that Americans in the public eye have.