However, before any of this could happen, there is one tiny detail that is always overlooked by supporters of the Fantasy Island idea: airlines would have to be persuaded to move, and that could prove difficult.
This is one of the many reasons why this is not a good business investment.
So you build Boris Island and then designate Heathrow as a freight hub and Maintenance base. Any freight brought in on passenger A/C can be sent by rail to Heathrow.