Manston is miles away from London
Gatwick is too far South to be of use to anyone North (Birmingham would benefit though)
Luton is too small
Stansted too far east (and problems of its own in expansion)
Thames Estuary is not fanciful at all.
Birdstrikes / crashes - almost an irrelevance really. Canada geese flock around Heathrow and will flock around the Thames as well. Birdstrikes will happen - management of the problem will minimise it. London City sits beside massive marsh areas of Belvedere, Erith and Dartford areas, yet operates safely.
The train network is connected to the Maplin Sands area through the new HS1 link; while Heathrow undoubtedly needs high speed rail to remove domestic flights that are inefficient and clog up the airport, a rail line going only to Heathrow will be uneconomic as compared to a used line going to France and Kent already.
I for one disagree though with the issue of crashes over the City - that too is I believe an utter irrelevance derived from the days when people did not understand engineering of aeroplanes. They will always fly over populated areas, and though occassionally disasters will happen - most flights are just fine. Disasters such as Lockerbie might still occur today over cities even if all airports were in the sea.