If the weather of the first quarter of 2012 in south-east England is likely to be more typical than that of te second quarter (hosepipe bans rather than floods), can the hinterland of Heathrow support a large enough population to justify multiple new runways? Users of Boris Island would face the same parched future.
That argument aside, the additional flights are needed now, rather than in ten or fifteen years time when either of the south-east options would come on stream.
Heathrow, despite climate considerations, is likely to remain the major UK hub, and the best origin for UK flights to destinations that require no more than a couple of departures by UK airlines per day. Well before we rach a dozen flights per day it shoud be possible to fill 777 or 787 from somewhere like Brum or Manchester.
There are suggestions on another thread that there is already a growing amount of transfer traffic between trans-atlantic and middle-east flights at Manchester. Given the necessary delays in introducing more capacity in the London area, is a viable secondary hub likely to evolve before either Heathrow runway 3 or Birdstrike Central become operational?