This question has already been asked a few times I think.
I also assume that you are referring to the N900?

potential differences between US and UK versions:

- Wifi channels are more limited in the US (1-11, instead of 1-13 in Europe and elsewhere)

- Warranty: a device bought in the US will be serviced only in the US. Similarly a device bought within European Union (+ Switzerland, and a few other countries) will be serviced anywhere in the EU

- different power plug

- prices shown in the US are without taxes...and you should also add the import taxes for your own country...

Apart from that there are no other differences that I'm aware of.

Link to thread with the same question:
http://talk.maemo.org/showthread.php?t=33495