Anyone thinking "God its Virgin" need not worry as Virgin don't have a mobile network to destroy (like their cable/ADSL network)
Virgin are a virtual operator and all services are provided by T-Mobile.
As an aside I've found that if you phone up any real operator and say "Hi, I'm on <competitor name's network> and I'm not very happy with them, can you send me a free SIM card" then they are more than happy to do so
They usually don't preload it with £5 though
Edit - I did this because I travel to places nearer the Arctic than London so having lots of SIMs usually helps find some coverage.