I teach at Southend. There is one thing wrong with that picture. The Southend Pier extends about four times further out into the estuary! Looking at the picture I can't see how shipping would be able to get up the Thames.
Second problem - from where the picture was taken, is a massive wind farm, that would cause major interference to ATC radar.
Third problem is that there is insufficient numbers with regard to the local population in North Kent to support a major international airport.
Anyone who watched the Commons Select Committee enquiry into the idea of an estuary airport a couple of years ago would realise this idea is a nonsense.
Finally how is it going to be paid for?
Once Mr Johnson is no longer Mayor of London, this ridiculous idea will die a death.