Most chargers are OK, but check that the label on yours says something like "Input: 110V-240V". The US operates on 110V, while the UK operates on 240V (the rest of Europe operates on 220V).
If an electrical item expects 240V but only is supplied with 110 it will either not operate at all, or work slowly (eg taking longer to charge batteries, or hair dryers not pumping out much hot air). However, if an item expects 110V and gets 240 it can have disastorous effects, so using US electrical equipment in the UK should be done with a large measure of caution.
Regards
Rob