I thought Boris was Mayor of London, not Essex...or Kent, for that matter?!
This is all clearly about votes, lets not kid ourselves. Boris wins votes around Heathrow by opposing expansion, whilst promoting expansion at places he has no jurasdiction over and have no impact on his reelection or otherwise.
Simple politics really, politics that Cameron and co should shrug off and do what their job is i.e. what's best for the country, not what a few people living round Heathrow think