Airscrew
There is no vacant land west of LHR that doesnt thave reservoirs on it or adjacent to it. Can't move those and they are not exactly compatible with aviation. Also that area is very very prone to flooding .
And thats aside from having a N-S Rwy operting across two parrallel east west ones or the conflicts with Luton and Gatwick from planes launching to the North and South. Could not see that working tho those who know like HD may tell me I am wrong there.
The estuary case wont be helped by yesterdays events in New York . Mayor Boris might be an idiot in some ways but he isnt going to christen himself Boris the Bird Butcher is he. Also how long would it take us Brits to build a monster airport in a river estuary without it running three times over budget,sinking , being built somewhere its foggy 150 days a year etc etc etc.
Sadly the Rwy 3 option is all there is and its no coincidence that the great big vacant space due north of LHR has stayed a great big vacant space for the last 40 years is it.
PB