If the airport infrastructure around London doesn't expand soon, the role of Britain in the world economy will shrink as functional airports in Paris, Amsterdam, Munich etc undercut our role. Decades of appeasement of myopic NIMBYs will soon reduce this country to a second rate economic power and that is very sad for its people including me .
It is Heathrow and Gatwick that need the runways. Some stupid jerk signed some deal years ago that said there could not be another runway until 2016 or something. Good thing they have the second runway or they would be screwed but of course only one can be active at any one time. They need to have two useable runways. Putting one to the north would be easiest since the two would sandwich the terminals.
But looking at the map, the flight path of this new 26R/08L would be low over Horley so they'd kick up a real fuss. What would be better is if they changed the runway direction to 27/09. That way they could build a new one to the north of the field. It could even just be a shorthaul runway (for now, you can always extend such as is proposed for Luton), which with a new terminal for EasyJet, who are massive at Gatwick, would take a lot of traffic away from the south runway.
The south runway could be replaced with another long haul runway parallel to the north one. The new south runway could be built a bit to the south of the current one allowing the current one to remain in operation while it is built as well as allowing the flight path for the north runway to avoid Horley as much as possible being not being too far north.
Then there's Heathrow. The proposal for a third short haul runway to the north of the current area could be useful as well as a T6 in between them. Heathrow definitely should junk that stupid Bermuda II agreement that says one runway must be used for takeoff and the other for landing, and switch to mixed mode. That will add a few slots and enable the airport to at least do as much as it can. Moreover, if 09L is being used for takeoffs, then aircraft from T4 have to cross 09R and taxi to the opposite corner of the airport wasting a ridiculous amount of fuel. Not even the most evangelical environmentalists can justify Bermuda II.