A bunch of those unlocking sites are scams, some are OK, but how to tell? Some will charge you, and tbh, I'd be suspicious of any that are free.
Just pay the fee and get it done officially by O2, then you're not risking bricking your phone.
You could root it and all that, but if you don't know what you're doing or how to fix it, you could have a load of grief and a dead phone if it doesn't work as expected.