The only reason Britain still has that reputation is because Americans repeat it mindlessly in media that the whole world consumes.

Like the teeth thing. In the 2000s, the UK alongside Germany had the joint healthiest teeth in the world (although now they’ve fallen to 8th after the Scandinavian countries upped their game). Did it stop the “Brits have bad teeth” gag in US media? No.

The US, for whatever reason, has been engaged in a cultural pissing match with the UK for a long time.