Inquisitive has hit the nail on the head but for one aspect. It is assumed that the options are Stansted and Gatwick while forgetting Luton.
Both Gatwick (from north of Greater London except the Thameslink axis) and Stansted (from anywhere except Cambridge) are rather inaccessible while Luton provides a better service from Central London ( and the Thameslink service from south of the Thames)
In my previous post I included the option of allowing all the four main airports in the South East to have two runways. The market would provide the answer to which would prevail. However, I always had in mind that the improved access to Luton airport (electifying the whole MML to Sheffield and Leeds) would mean a tremendous increase in demand for Luton (without having to go through Central London for most people). This would benefit both the bottleneck of London and the alternative services to Luton (including those on the Thameslink services from south of the Thames).
Think again and forget about Maplin madness or any other airport in the estuary.