What is your solution to the problem?
Runway 3 at LHR demolishing Sipson allowing the most bang for your buck in investment terms. It's a built up area anyway and the houses round LHR have been aware of the airport since 1947 and runway 3 won't markedly change that perception, there will just be no half day break from the noise.
LGW is almost but not quite green field, with lots of nice areas nearby and lots of rich, powerful and influential people who would go all NIMBY as this would be a significant impact of noise on areas thus far unaffected. I think this is pretty unfair, the same goes for Stansted.