The best idea is a high speed surface, probably rail, infrastructure linking all of our existing airports. In WW2 it was recognised that dispersal was a good idea. If it was arranged that the airports around London were reachable in about the same time as a walk between terminals, then much better use could be made of the existing facilities. Many of the provincial airports are capable of extension, and the increase in national capacity would be much greater than super overcrowding an already overcrowded bit of airspace approaching and leaving London.
Recently, I flew back from Cyprus on a late flight to Manchester. The matching flight to Gatwick was cancelled for "technical reasons", probably technically under-populated, the lucky folks going to Manchester and getting bussed to Gatwick. With a fast infrastructure, they would have been much less unhappy.
With birdlife around, airports on estuaries are never going to be a safe proposition. If, say, an Airbus finds a flock of geese shortly after take-off, most of the flock will get away with it. The Airbus, lacking engines, will not.