Can anyone here explain to me why this would NOT be the best and (probably in the long run) cheapest solution for the future of air transportation for London?
Sure.
Because there are already three airports with four runways which are capable of high-capacity, long-haul aircraft. Two such airports are capable of handling 24-hour ops. Both airports have sufficient room for another runway each (one already has a kind-of half-a second runway anyway). One has lots of room for planned terminal expansion. Both have existing road and rail links into central London which could easily and quickly be upgraded to a world-class service. Both are closer to non-London catchment areas which are at least as big as that around Canvey.
Whilst both airports are well-established, one has left its expansion a bit late such that there are now zillions of nearby residents who would scream blue murder at further major expansion. The other has lots of open land and less locals around it and could expand before it gets built-in.
If the pollies had the cojones to get the ball rolling and made it cost-feasible for a few majors to go there...
Actually while we're at it, all this talk about EGLL getting a 'third' runway - it used to have
six (and until quite recently, still had a 'third' - 23). OK, it was a cross-strip, but a bit of decent co-ordination and it could relieve a bit of pressure on the other two, surely? Eg. launch some RJs and 319s off it? Even extend the T/O starting-point to the north-west of 27R to get clearance over the buildings to the south on warm days?