Gatwick is in stockbroker belt and the demand for the second runway is at Heathrow, as are all the major airlines that paid millions for slots to get out of Gatwick. The people that live near Gatwick are very influential and will in my view manage to stop plans for any second runway.
Gatwick and Stansted are holiday airports, Heathrow is a world hub that needs good connections. Landing the Glasgow shuttle on a short runway at Northolt and then making a five mile connection isn't going to win you high yield business passengers.
There are good reasons why these ideas were dismissed and they have been debated before alas. A proper third runway at Heathrow should also insist upon the return of connections to Jersey, Guernsey, Inverness and other parts of the UK domestic framework that have lost their quick link to the world.
Crossrail will happen, it HAS to. The Central Line is killing me slowly (!)