As a TMA controller, perhaps I'm being a bit blinkered, but let me set a scenario. A Tuesday morning in November, all Southern England covered in fog, best RVR for hours 250m (perhaps putting another runway on top of the hill would help?). All holds full for all airports. Any extra capacity? Well, no.
OK that's a 'bad case' scenario. Let's have a better one. Tuesday morning, bright summer day. All holds full for all airports. Any extra capacity? Still no I'm afraid.
The point is this. There is extra capacity in the ATC system. It's generally between 10 and 1130, 1430 and 1700, and then after 2130 on weekdays, Saturday afternoons and Sunday mornings. Strangely, there still seems to be nobody catching on to this. Having extra runways in the London TMA will help a little, but things are still very crowded in the air and I don't believe you could get many more up there at peak times.
More airspace would help, but 'special interest'groups, the airforce, and the fact that we have crammed 5 major airports into the smallest space possible all make it a little difficult.
If there is to be a strategic long term plan for increasing runway capacity in the S East, perhaps we need a total reorganisation of the airspace? But when did what we need actually matter.....
I can only hope that if/when the government comes up with it's grand design, some thought is given to those of us trying to get the planes to and from these new runways!