Im not gonna throw morals at you regarding this as you will act upon your own judgement and standards anyway. Your obviously not going to hand it in as you have had it in your possession for this long.
If the iPhone is on contract then the uk owner would have possibly reported it to the uk police in order to obtain a crime number so they can claim on any applicable insurance.
02 won't unlock it for you as its not registered to you.
It won't be discussed on here on how to bypass lock codes or other methods so you can reset it and rightly so.
I recently had my iPhone 4 stolen and am only too glad i imei blocked mine, remote wiped and locked it using the findmyiphone software, and had app locker installed to add extra security on sensitive apps such as banking etc.
I assume the thief ripped out my sim card.
I would like to believe in karma with what goes around comes around.