A complete lack of all the Native American/Incan/equivalent factions, and therefore possibly characters. However, many more faeries and such. As previously mentioned, weapon laws are vastly different - no guns, knives, swords, or anything that could be construed as a weapon (make sure you hold onto your nunchuck receipts on the way home folks, and yes, there is a story there).
Cities are built differently - many American cities are either built up from the native civilizations mentioned above, or created when the European settlers came over. British ones have developed for much longer than the settlers'. Building styles will be different, town layouts will be different, and so on.
No constitution. We have laws, and they're built on... I actually don't know, but the closest thing we have is the Magna Carta (I think, never was a history guy).
CASTLES!
I remember having a conversation on here a while back with whoever it was had a lion for his DP about holidaying in the UK. Distances seem bigger, as we have a smaller country. This means that we don't have as awesome railways and such. We also have a longer (written) recorded history, being Europe and so on.
We also have nice tea, but our coffee (for the most part) is near undrinkable.
Umm.... That's me out of things I guess. Hope it helped.