Boris island is a ludicrous idea. You have people travelling all the way from west of London to further east than Southend to fly long haul... the whole infrastructure surrounding Heathrow and the m4 corridor, and all the business located there will have zero interest in that ..
more by luck than design there are three/four international airports around London. That isn’t going to change .
the rise of the regional airport will continue for a while yet, especially with the improvements in airplane performance