I recently did two iPhones. If your not the original owner then it will cost £15. The easiest way was to buy an O2 pay as you go sim with £15 credit on it. Don’t sign up to a package, just £15 credit. Call O2 customer service and follow the menus.
Ask to unlock the phone, you will need the O2 SIM card in the phone for at least 24 hours before you do this, the phone number of the O2 sim and the IEEC number off the phone, in settings, general, about. They will use the £15 credit as payment.
O2 will send an unlock text message, then take out the O2 sim, insert any other sim, plug it in to your computer and sync to iTunes.
iPhone unlocked.
I found out all this the hard way, nobody will tell you all of the steps, so I’ve done the hard work for you.