Maybe their lack of dental hygiene is a relatively new phenomenon.
Actually, the chronically bad teeth of the British, came about in the 1600's when large amounts of sugar began to be available.
People on primitive diets - including the more ancient inhabitants of Britain -- often have excellent teeth (no crookedness, little or no decay) despite a total lack of dental hygene. Even as late as the 1930's there were isolated Scottish islanders, living on seafood and oats, with perfect teeth -- but as "civilized" food reached them their teeth (and general health) were swiftly ruined.