Virgin Atlantic are a strange airline really. Firstly, they only operate long-haul flights, which isn't a common business model at all. Secondly, they don't belong to an alliance and rely on codeshares for connecting flights, so most of their market is Origin & Departure (i.e. people travel just between, say, London and New York). Thirdly, they operate many holiday type flights from Gatwick and Manchester to places like Orlando and the Caribbean, and that has been their main growth area.
Basically, without Virgin Holidays on 'sunshine' routes out of Gatwick and Manchester, I don't think they'd be able to support their long-haul flights out of Heathrow against fierce competition. For that reason, I think that BA becoming stronger at Heathrow will do them no favours.