Richard Bransons Virgin Group initially had around 10% ownership and Richard Branson was in fact the largest shareholder of NTL Telewest Group 2006. The deal was also to use the Virgin name replacing NTL Telewest for long period of time on terms of a lease.
NTL and Virgin actually go back to 1996 when they both launched virgin.net
And Virgin initially used Mercury for all there businesses, in fact Richard Branson himself once featured in a Mercury TV ad in the early 90s(see link below), that is a sign he wanted to get into the telecoms business! He's always used Cable and Wireless and NTL, One2one at the time of launch of Virgin Mobile was still owned by Cable and Wireless. And yes Mercury was Cable and Wireless and not NTL, but NTL did eventually take over the residential 131 and 132 dialler service for a while so literally making it all the same.
Virgin.net became a wholly owned company of NTL in 2003, first step towards the making of Virgin Media?
Funnily enough all those Mercury payphones disappeared 3 years later to become Interphone bright and orange kiosks.
But back to the subject, Richard Branson has wanted a piece of telecoms market and now he's got it, he doesn't need to own 100% of it, when it comes down to it Virgin Media is a PLC and Richard Branson is one of the largest shareholders, whilst Virgin has there name branded across NTL Telewest, they are involved! Although I think they should have more involvement in what goes on than they already do, they don't necessarily need to own 100% of it to do that!