yep, it'll work. Just like "StayingAnonymous" said. I'm in the opposite boat....I got an iPhone over here and am taking it back to the USA. Same procedure....get the carrier to unlock it. They shouldn't give you a hardtime since you are moving overseas.
AT&T unlocked our old phones when we first got here. We did it over the phone while we were here.
Before you buy the iphone...specifically from AT&T...make sure you check that they will unlock it for you. We come over a little over a year ago and they refused to unlock our iphones even after we explained we were military moving overseas. We called AT&T and we called Apple and no one would do it. My uncle is vice-president of AT&T and even he couldn't get clearance to help us unlock them...we even got correspondence with the president's office. Nothing. Just check before you do it.
You can unlock your iPhone yourself. There are several good programs available online, just have to search around a bit. My iPhone was jailbroken and unlocked within an hour of purchase. There are also some people that advertise on this site for around £30 to unlock. Personally I think that's a bit steep for something that takes all of 5 minutes.
HI yes you can buy iphone from USA and use it in England only after unlocking it .If you want to unlock your iphone then visit Mobileunlocksolutions.com here they unlock iphone using remote unlocking service .After unlocking your iphone you can freely use it with any GSM sim .