@slowwalker: There are two things that could prevent this. The first is that the network in the US is predominantly CDMA and not GSM which is used almost everywhere else. The second is that phones can be locked into the domestic network eg verizon or AT&T. Phones can be unlocked but if they are CDMA then they simply won't work. You will need a prepaid sim and a world phone that works in the uk and throughout Europe.