Have you checked to see if the phone is actually locked? If this was a contract phone as opposed to a PAYG then it might well have been supplied factory unlocked to start with. This has been the case for some years.
Try a SIM from a different network, doesn't need to have any credit on it. Best to use a SIM that is not EE based. If the phone is locked it should prompt for the unlock code. Don't try and enter anything, just switch it off.
You should also view the £15.32 as a limit. Shops that unlock phones can be found on any high street or market. What's the going rate for your make and model? It might well be possible to get the code on ebay. Pick a vendor with high volume and good feedback who state they can unlock your make and model from Virgin UK. I've unlocked a variety of handsets from as little as £2-£3. The code is emailed within a few hours.