With regards the climate change obligations, it hasnt seemed to rear its head as a problem with people proposing the new 4-runway estuary airport, so one extra at LHR and LGW instead of the alternative of 4 extra and closing 2 (i.e. LHR) shouldnt be a problem.
In addition to that, we know that aircraft are far cleaner, quieter and more efficient than they were 10 years ago, with the 787, A380, A320neo, 737 Max and probably A350 all being either re-engined or carbon fibre construction (or both). Airlines are also seriously experimenting now with biofuels, I know Virgin and Lufthansa are but two, so that could show fruition in the next decade. This is all in addition to the constant little amendments being mooted such as electric motors on the landing gear to negate the need to the main engines to push aircraft round the taxiways.
Obviosuly I'm not an authority on the issue, but it seems the airline industry is seriously trying to reduce its carbon emissions...whether you think that is for environmental reasons or simply to become more fuel (and cash!) efficient is neither here nor there in terms of the outcome I suppose.
With regards the layout of LHR we all know it needs rationalising, and work is underway in order to progress that, though will of course take time.
With regards Brum, I think they need to concentrate on fully utilising their current runway capacity before making serious arguments for a second. Manchester seems the more obvious location for a hub of sorts, but would again require a large airline tenant. No signs of that being BA, so the alternative would be either Virgin or even one of the Middle Eastern airlines...Emirates already have a large presence there, so why not I guess?! My only suggestion would be that it would only work if LGW (or even STN) was not to be used as a secondary hub i.e. with 2 runways, as to have three 'hubs' (or one, and two smaller ones) would seem overkill. The closest comparison would be Germany (Frankfurt, Munich and what they are trying to do with Berlin), but I think we need to wait for the economy to sort itself out before justifying that!
And Skipness, I know nobody has ever made the transition before, but the requirement has never been as urgent as it is now. EZY have no intention of doing anything other than point to point, so can continue as they are, but I think to rule it out simply on the basis that "well, it didnt work in the 80's" seems like it should be better qualified to me.