Walnut,
Some good points here.
However not discussed here is how Gatwick would pay for a 2nd runway certainly the low cost airlines wouldn't.
At least at LHR with fatr more full fare airlines and business/first class passengers the revenues are much higher.
Financially it has to be LHR, thats where business passengers want to fly from.
However with little political appetite to expand LHR I can see them going for LGW in the long term .
Nigel