I believe easyJet have already ruled out LHR recently quoting cost. Also given the choice between BA and EZY, I suspect most would pick BA for connections given loads of baggage involved and the lack of boarding scrum. Long haul is stressful enough. There are no likely candidates for franchising either, BMI Regional flying ERJ-145s feeding B744s and A346s on long haul is not ideal.
What about Air France/Cityjet going for the slots they seem to do pretty well will LCY from EDI.
They've gone from the dominant carrier to a poor second as they allowed BA CityFlyer to trounce them, albeit with the help of RBS. CityJet are not in the best of shape financially either, so unless Virgin want to use them as a franchise with a subsidy to offset P2P losses (
again - happened on LCY-DUB in the mid 90s), it's unlikely.
You know what? Anything is possible and I suspect someone might give it a go, however it would be a triumph of hope over past experience if they could make it pay.