Thames Estuary did not proceed for the simple reason that not enough pax live that side of London and all the ground routes lead neatly into EGLL, due to history.
The minority report from the Roskill commitee opened with the statement that made that very point and recommended that Cubblinton should be the site, not the Maplin sands, for the 3rd London airport. However I have a feeling that nowadays Maplin is about the only place that would stand any chance of
being acceptable, and of course would have the major advantage that it couldn't be surrounded by houses.
Certainly a considerable amount of transport infrastructure would be required, but with maglev trains capable of reaching 300mph+ a suitable system could easily be designed that could make it viable.