MAN-LGW would work for either EZY or RYR if they so desired.
LGW is IMO the best London airport to fly into as the train will take you into central London to Victoria or London Bridge. I don't really consider Paddington (off the Heathrow Express) to be central London.
The trouble with Virgin trains is that there is no competition between London and Manchester and therefore prices are eye-wateringly high in the morning. Someone like RYR could compete on price even when the last bit into central London is (ironically) by train. I would travel to London more often if there was a cheaper alternative than Virgin Trains.
Slots at LGW would be the problem I guess.