It's a cultural difference, even when what the UK woman reveals isn't actually that extreme, it's still a bigger deal to an American eye.
In the UK this issue is just not as big a deal -- in very, very general terms yes the difference is largely owing to a slightly more "puritanical" leaning that has never really gone away in most of the USA, while most Brits, not all, but most, are more likely to think "so what?" to a bit of skin or boobs.