As one who qualifies as a frequent flyer which, as far as I can see, means only that I get more spam email than ordinary mortals, I think it's worth pointing out for the benefit of those who perhaps haven't sampled the delights of travel to and from our three major London airports, that the time element between each airport and central London isn't in fact greatly different.
It's true that Heathrow has the quickest route but only by the relatively expensive Heathrow Express non-stop train at a little over 15 minutes usually. Using the tube or bus from airport into town works out at roughly the same time as it does using the fastest connections at the other airports. The actual distance between each airport and central London becomes irrelevant really when the time difference involved in the journey is taken into account.
There are costs involved for airlines using Heathrow which deter many operators from using that airport and that's why the likes of Ryanair utilise other bases, keeping costs down. Even Branson has complained at the charges extant at Heathrow!
I take Dave Elderfield's point about the bird life on Dungeness but Heathrow Heath was not dissimilar when it was decided to up sticks and move London airport from the old Croydon Aerodrome. Today, the green brigade would probably prevent such a move on account of the wildlife habitat. What I find extraordinary is that people seem to count for so little as we have seen when Heathrow expansion was being considered. The same thing is happening with HS2 and the scheme's proponents seem to take notice when wildlife habitats are mentioned but totally ignore the potential blight to humans unfortunate enough to live near the proposed routes. As it happens and returning to Dave Elderfield's point, I'm not sure that expanding the existing estate at Lydd airport would have such a great effect on the bird life as the airport has been there for more than half a century already. Further over on the marsh, I believe Dungeness C station will never happen primarily on account of habitat issues so who knows?
I think it's important that we keep in mind also that not all travellers to and from London airports are London-bound or London-based. Many use one or other airport for different reasons.
Now, let's see.....where shall we build the next wind farm or nuclear power station?
Indalo