I'm not ATC or even professional pilot, but I am someone who uses the airports.
Heathrow is always busy, always congested, and except for the Heathrow Express, an hour away from the city centre.
I used to like Gatwick...but the South terminal seems to have become intolerably cramped over the last couple of years. And again, you're about 45 minutes out of the city centre.
Stansted is dull...but effective! Feels spacious, but again, an hour away.
Why not cap LHR at 2 runways, and properly commit to STN. A proper high speed rail link would cut the journey into London to around 20 minutes. And there's room, without upsetting too many people, for runways 2,3 and 4, and terminals to match.
Some genuinely strategic thinking needed...