It is good that the Govt have finally made a move on expanding LHR after about 30 years of dithering, but surely I can't be the only one to see how this is way too little way too late?
Lets say we have a runway built and ready to go around 2025, which given the commissions, legal challenges and bodged construction companies in the pockets of politicians that are likely to appear is not particularly unrealistic. Looking at the current state of the London airports, LHR will obviously be full as it is now, LGW is getting to the same point now, so will almost certainly be 100% full in 5-7 years time. STN is likely to be close to where LGW is today, and LTN won't be far behind that. Up north I would expect MAN to be in a similar position to STN, although maybe closer to LGW as the new terminal / second runway should allow it some extra capacity. So after LHR R3 opens I'd predict about 5 years of relief for the London airport system before the whole situation starts to repeat itself.
The Govt really should have let LHR, LGW and STN build their own extra runways if they wanted too, and let the airlines and passengers decide where they want to fly from (well aware that politicians are way too spineless to make such a move...). In that situation, yes they would not all fill up evenly, but there would at least be more competition, and it could be considered a long term move. LHR would be able to boost its position as a hub but also importantly LGW and STN would be able to grow to help cater for the ever growing O+D traffic, which lets not forget, makes up the vast majority of the air traffic to and from London.
As it stands now, as soon as LHR R3 fills up then I feel we will be back to square one, except with the possibility that STN and maybe even LTN in a position to present a justifiable case for a second runway as well as LGW, meaning even longer grass will need to be found this time around...