I have an AT&T iPhone 4 on which there is still about a year of contract. For the next 18 months, I shall be spending most of my time in England with occasional trips to the US. After some research, I think I should be able to unlock the phone and use a UK sim when I'm over there and slip the AT&T one back in when I'm here. I'd appreciate any advice (a) regarding pitfalls with that scheme, (b) a suitable unlocking tool and (c) ideas about the pros and cons of the various UK carriers. Thanks in advance.