The problem with the Thames estuary is that it is too far for everybody west of London.
For an airport to be successful, airlines have got to want to use it, as the Spanish have found to their cost. The airlines have no interest in expanding their operations in Birmingham as the main market and traffic is centred around London. Luton has plenty of spare capacity, but with its poor transport links, there is little interest in airlines using it more.
Boris island aircraft flow will cause Heathrow major problems and vice-vera where London is common airspace. There are many problems with the Thames estuary, tree huggers will have field day, sunken WWII explosives, not to mention major bird strike risks. The problem is where the third runway at Heathrow has been consistently ducked by successive Governments, Heathrow is already operating beyond safe capacity. All takeoffs and lands are at the absolute minimum wake spacing, so there are no margins of error.
My guess is that the arguments will still be going on in 10 years time and will have been solved. Anybody with an international business would be better off locating it near Dublin in the ROI or near Schiprol in the Netherlands or Munich if it is financial services related. So once they have all done that, then there will be no for a third runway or new airport capacity and where we have mass unemployment we can get them doing something useful like tree hugging.
15 years a go, I was flying regularly to Paris on business from Heathrow and it was an absolute nightmare, as fog in the Paris basin is a common Autumn / Winter / Spring problem but not a problem with Cat III landing systems apart from the aircraft spacing has to be bigger, a few minutes delay at Charles de Gaulle not a problem, missed takeoff slot at Heathrow, always meant anywhere from a 1 to 3 hour delay, so an all day 10am meeting would not often start until the afternoon. The Chunnel was a non starter where it was slower., the only practical solution with this was to go the day before have the extra cost of a hotel and 1.5 days out of the office, instead of one.
The UK transport infrastructure is a joke, one that is costing the UK billions in lost business.