That phone is a quadband GSm/triband UMTS. So, it should work anywehre in the world there is GSM. Unlock it right away since you already have the code. Then put any other sim (active or not) that's not AT&T's to verify that it's unlocked. Then like others said, get a prepaid sim when you get to London. There are several optins that have already been discussed here. Just hit the search button.