I think that the fact that you decided to use the �3 of credit on the sim card, indicates that you intendede to permanantly deprive the owner of the sim card of their credit. This by definition does make you a thief. You could have handed in the sim card to the chip shop or the police. You could have contacted O2 and told them that you have found this sim.
If you want to get the phone unblocked ring your phone company - explain that your phone is locked, and see if they can help. If you are asked to explain further you will have to explain what happened.