Can I use my US iPhone 4S when I move to the UK by just getting a new SIM Card?

I'm moving to Scotland in September 2013 from the US. I have an iPhone 4S with AT&T, and I was wondering if, when I arrive in Scotland, I could sign up for a mobile plan with a carrier that supports iPhone and use the phone I already have. In theory, they just have to switch the AT&T SIM card out for theirs, right?

You need to get your IPhone unlocked this will allow you to use it with any GSM carriers that use sim cards. If you need it unlocked AT&T does it for free now you can see the qualifications to get it unlocked on their website https://www.att.com/...ent/en_US/

Unfortunately you can't. My friend went on holiday to the USA a few weeks ago. He couldn't use his UK iPhone over there. He was told there's something to do with the signalling. They have slightly different parts in them. So I guess you might need to buy an iPhone from the UK!

EDIT: However, it is always worth a shot! Your best bet would be to ask Apple. They would be the best ones to ask. I don't see how it could be a problem!

Yes, but you forgot one part. You would have to get the phone unlocked 1st & of course cancel the AT&T contract before you leave.