How much would it cost me to unlock an iphone 4 in the UK?

They will charge you 20.42 and can take up to twenty days depending on the phone and who you talk to there number is 150 from the handset or 07953 966 250

The original owner can ask their network to get it unlocked - it's usually free for contract phones, possibly £15 for pay-as-you-go. iphones do not use an unlock code or anything like that, Apple send an update to the phone via itunes which modifies the firmware in the phone. There is no other way of getting one unlocked than via a phone service provider. If the original owner will not do it, try getting a pay-as-you-go SIM for Orange, and ask them to link the phone serial number to your account. There is a possibility then that you can request an unlock via that company.