Silver,
If you can get the locals to agree to a 6-runway airport I might agree
I never said 6 runways - they are nice to have when the room is available, but the only way you will get such a facility in London is to consider all the London area airports as a single entity.
I remind you - we can all have our ideas, but the only option actually on the table at the moment is the one proposed by Lord Foster, partially on the Isle of Grain. So although it will have 4 runways, 2 of them replace LHR, net gain two runways. As already discussed, net terminal space gain around 50% compared to Heathrow.
To build 3 x 2 parallel wide spaced runway pairs on that site would effectively mean damming the Thames. That is the sort of stuff for Sim City geeks, not a serious proposition.
You say LGW would have problems with planning but there would still be incredible objections to your project from environmental groups.
Now you might say build further out - fine, but the Thames Estuary is not Kansas prairie so the cost is going to shoot up even more, over and above the 1:2 ratio you already have to deal with of capacity increase to Heathrow replacement.
I don't for one minute think extra capacity at LGW would be an easy sell, but take a look at a map of either LGW or STN, look at the number of houses underneath the approach paths and then compare that with LHR.
Then look at the political map around LGW - all blue, so a Tory government could piss on their own doorstep and be unlikely to suffer major seat loss. And do you think a Labour government would be bothered? On the other hand, LHR is a double-edged sword - allow a 3rd runway, and you would almost certainly lose any marginal seats. Even worse would be to close LHR and cause such massive relocation - LHR is the airport people love to hate, but watch the uproar if you
actually proposed closing it.
I think the much bigger problem for LGW would be one of finance. The airport was bought iirc for £1.5bn, so to add a new runway would probably mean an investment bigger than what was spent on the airport itself. Ditto for a new terminal of the size needed to benefit from the new runway (25-40m pax pa) and create a hub operation. Then you'd have to
tempt someone from LHR.
But all of this is pocket change compared to what is being proposed on Foster Island - which is more like £30bn compared to £3bn. And you'd have to
force all airlines at LHR to move there - simply far too tall an order given London's disparate airports market and the realities of UK politics.