@Highwideandugly
That would indeed be the logical solution, but unfortunately it's too much of a political hot potato. I'll let everyone make up their own minds on that, but my 'at-a-glance' take on it: It makes sense to add millions of extra seats across the Uk and make better use of these infrastructures rather than concentrate it all on 1 airport. On the other hand, airlines, particularly long haul, won't want to spread demand across more bases/destinations in close proximity, as it dilutes their yields, compared to focus on a couple and being able to charge an arm and leg. THey would prefer the expansion at LHR (and a couple others).
Try telling the politicians all this though and see how far you get...