I don't see why VS or U2 would want to get in on this market
Virgin have codeshared with BMI on UK domestic routes for many years, it's an important feed for their LHR longhaul routes. They are now in the strange situation of having their codeshare on the flights of their arch-rival, BA.
Clearly they won't want to gear up to operate shorthaul routes themselves, but I wouldn't rule out a franchise operation or partnership with a new entrant carrier, or even a VS/EZY codeshare.