As i understand once you have topped up with £30 (thats actual top up not what they give you) and you have had the phone for a couple of months they will unlock it for free. If not various websites will tell you how to find the unlock code.
Simply make a top up of £30 or more, give them a call and they will unlock it for you. No need to wait , do it the minute you top up with £30 , keeping in mind thought that you'll be better off using up the remaining credit with Tesco first before doing the transfer, or even cheaper, just use an online unlocking service. Good luck