Actually, the belts are NOT the same. I know this b/c my UK family have had difficulty installing UK infant seats in American cars, as our rear seatbelts are generally not long enough. That shouldn't be a problem for you using a US seat in a UK car, however.
When installed without an ISOFIX base, UK infant seats use the shoulder portion of the belt as a sort of top tether. When you install the seat rear-facing, the shoulder portion of the belt wraps around the back of the seat shell behind baby's head, and the seat has a channel for that belt to route into. (Therein lies the problem with the belt length, you see.) Here's the only photo I could find to demonstrate:
http://www.babysleepcentre.co.uk/ima...ear_facing.jpg
Now, that said, I can tell you that we have used our US seats in UK and Eire hire cars many times, installing them according to the mfr. instructions, and have never been called down about it, except to be told by the rental agent that carseats should never be installed in the center of the rear seat. (I've been told that the reason for that recommendation has to do with the general lack of intersections as opposed to roundabouts; a difference in the more likely impact point.)