Unlike many other providers, Virgin don't lock the mobile phones they provide to their own network. So you can switch networks very easily if you want to. If you choose to do so it's easy to retain your old number. (You simply ask them for a 'PAC Code', which they're obliged to give you upon request, and give that to your new service provider).
So irrespective of whether you stay with Virgin or not, (or of whether you take out a contract or use pay-as-you-go), you can ALWAYS keep your old number.
At one time PAYG services could workout far dearer than contract services but, these days, more and more people are opting for PAYG through using 'bundles' which (because of the flexibility they offer) can actually workout far cheaper than contracts. So that might be worth looking into for you. (The most popular services among ABers seem to be GiffGaff and Asda Mobile, which I use myself)