i was asking a similar question about moving from orange to virgin.
http://www.cableforum.co.uk/board/93...in-mobile.html
looks like virgin use the t-mobile network.
virgin mobile and virgin media are seporate company's so it won't really get you anything, but the rep on phone maybe able to do a deal for you, have a look at the posts here
http://www.cableforum.co.uk/board/93...le-market.html