IMO Boris Island/Thames Estuary will never see a hub airport.
LHR (sooner) and LGW (after 2019) + one other will each have an additional runway plus facilities.
New technology and transport enhancements will remove the need for any new airport as I predict :
– High Speed Rail across Europe reducing short haul traffic, opening slots. Direct HS rail feeds to other Hubs eg Schipol.
– Engine technology enable noise levels to continue to fall allowing for a longer window of flight activity around LHR and others.
– Aircraft materials technology significantly reducing weight allowing for shorter runways to become full utilized and the norm.
– As the volume of pax increases the corresponding increase in capacity will be mainly satiated by larger aircraft as opposed to more flights.