How to connect to Network Carrier in London, England?

I have an IPhone 4S and my regular network carrier is AT&T, I'm traveling to London, England for a few weeks and have actually just arrived in the lovely country. Everything is wonderful and I have connected to Wi-Fi, however, I can't seem to connect to any network carrier. I've tried multiple ways and I have the option of which carrier to select but when I select it, nothing happens. I'm wondering if anyone can help me. I need to have full access to anything and everything on my phone for family and business purposes while I'm here.

I suspect you have a block on international use either on the phone itself or the account? However, if you will use the phone much here, you will be probably be much better off buying a UK 'pay as you go' sim card from a phone shop.

Your account has to have international roaming turned on so odds are you don't, anyway it will be much cheaper to just buy a pay as you go sim card, if you look for carphonewarehouse or any of the supermarkets the sims cost about £1 from most then just buy a topup to add credit. You will be best going to a proper phone store if your unsure of the frequency your phone can use.

The internet is transparent so there should be no reason why you could not connect to AT &T here as well it is just a matter of a request.via most carriers here as long as the phone can make calls this should not be an issue.