As I understand it one of the rationales for this airport expansion is that the UK is losing out because it does not have a decent expandable hub airport.
I have just read that DXB is about to build a second airport. Clearly its not needed for the local population so therefore does it matter that the new London hub airport is actually close to London.
On this point Boris Island does have some merit.?
Er yes! it's a hub airport, so it has point to point pax AND transfer pax. The presence of potential transfer pax makes some routes viable that would not be purely on point to point traffic. So they all have to be at the same airport, i.e. Heathrow only. It really isn't rocket science.
Boris Island makes no sense on ANY point.