Only way to unlock that is to install the ipad baseband on it with redsnow. You will lose GPS if you do this though.
If you want to do it I would first suggest downgrading to 4.1 as that is a untethered jailbreak.
So you would restore to 4.1 in itunes, after getting 1015 error use tiny umbrella to exit recovery.
Activate the phone normally with the O2 sim first. Then use redsnow 0.9.6rc8 to install cydia and the ipad baseband. After that you can install ultrasnow to unlock.
Be aware once you install the ipad baseband there is no going back though. You will have to restore to custom firmware every time after that if you want to restore/update.
Your better option since it is a O2 phone would be to get O2 to officially unlock it for you. Much safer and with that way it will always be unlocked and you can update without issues.