It's beyond me why anyone would want to travel by air to London (if you're going to the city center, that is).
Granted there's no low-costs from MAN-LON, but if Easyjet etc were to restart LPL-LON then I can't see loads being decent at all.
As said many times, it's probably quicker to train it dahn sarf and you've also got to take into consideration luggage etc. Flights alone would be more expensive than the train, nevermind adding a bag to that too.