Let's suspend diss-belief for a moment.
The method of constructing airports surrounded by water is understood, tried, tested and works. Land raising is a muck shifting job until you want to do something with your island
Putting a multi-runway airport with all the necessary infrastructure and integrated transport links has been done both on land and sea.
Designing airspace with STARS and SIDs is a matter of following ICAO Doc 8168 taking account of existing airspace, geographic constraints and noise. NATS and DAP can do this at a world class level. RNAV/GNSS ops can only help.
But (and it's such a big but its bigger than Vordeman's) there is statutory method to compel commercial entities to move from one airport to another. Airlines based at LHR have, over the years, made huge financial investments.
In order to close Heathrow, or at least down grade it, the compensation owed to those entities has to be factored into the Boris Island project. How will this happen?
Furthermore, the construction is a PFI so the burden of paying airlines to move will push the return on capital employed beyond economic reality.
So can we consign Boris Island to the same backwater as Cliffe, Maplin Sands at Foulness and Cublington (Wing) purleeeze.
Even Boris now seems to accept that another runway at Stansted or Gatwick would be cheaper and viable.
Lastly, what about tearing up the Cranford Agreement (not that exists on paper) Without alternation and the night flight ban there is substantial capacity to unlock.
Mixed mode on both runways has been modeled and with quiet a/c doing CDFA (controlled descent final approach) even a couple of extra hours would help.
Whatever is decided it will be too little too late, except Emirates have the right idea - direct services from regional airports. Southend is next
SGC