It is more to do with international connections, people do not want to fly into LHR then journey to STN/LGW etc to continue their journey.
Even with the airspace question though I would question why this is in GA?

Robin
I argued the case for Greenham Common many years ago on another forum. It is ideally located for where business people live, work and want to fly from. And of course it is people paying to sit in the front end which is attractive to the airlines.
This is why Manston/Boris Island is a non-starter unless you want everybody and their businesses to vacate the M4 corridor and up sticks and move to Kent.
Trouble is, like everywhere else that is proposed, the local MPs are cr*pping themselves about loss of their seat if they are seen to support it coming to their area.

Indeed in 2009 about 15% of Heathrow traffic was transit (same plane in and out) and 40% was transfer (change of plane). I see no reason for this not having increased or at least remained constant. So more than half of the customers for sure do not want to have to drag to another airport.