What about Manston?
Very large runway, close to a major rail route and a motorway.....
Manston/Kent Intl has tried to run airline services, and I flew to Nice from there once, perhaps twice on EUjet. KLM fly to Schipol twice a day at the moment.
The problem is that North Kent isn't actually very close for that many people. Upper Heyford makes more sense, but neither will ever happen as London's third airport.