They used to be separate franchises until ntl swallowed them all up. All of those franchises were saddled with debt from their network rollout. If there was competition they'd have to do exactly the same and we'd have it all over again. It's an expensive game laying cables so actual competition on the scale of Virgin is just not going to happen.
Of course there's the argument to open their cable network up wholesale but I don't see why they should, private investment put that infrastructure there unlike BT who were nationalised. Besides, just like BT (in most cases) they'd be running the physical network anyway.
Biting the hand that feeds IT
