Gift gaff which runs on the o2 network has much better deals. Www.giffgaff.com 10 quid will get you 500 anytime mins, unlimited texts, 1 gig data and free calls to other giffgaff numbers.
And you can keep your number the same by porting it across. Plus if the phone is locked to o2 it will still work
Virgin mobile , but if you leave and request your PAC code they will match the deal within a few days.