Fairdealfrank.
Boris Island is surely unviable for many reasons, firstly many of the LHR workers live around the West . Wouldn't be too far for many to travel if its way east ?.
Perhaps the only viable option would be to have a 4 runway airport to the west or north west of London. You already have road and rail connections close by and the workers could stay put.
However Im not from the London area and have no idea if there is such a location with enough land free ??
Nigel