Unlocking a phone on Virgin Mobile costs £15.32 whether you're on pay monthly or pay as you go.
For security reasons, Virgin require you to register the device you wish to unlock with them before they carry out the service. You will also need to pay all charges owing on the account before they can unlock your phone. The process can take up to 30 days.
You will need to call the Virgin to arrange an unlock as there's no online form. The details are:
0345 6000 789 or 789 from your Virgin phone. Also refer to their tariff page and terms of service for more information.
Visit Virgin Mobile for more information on how to unlock