Im with Skipness on this one.
Why do people think it is an easy thing to do to just relocate Heathrow! Look at the airport area on a map and all those factories, hotels, warehouses etc are all serving the airport. Do we just relocate those as well? What about all the workers? Mass migration to Norfolk/Kent plus construction of a huge airport would cause more protests that the third runway. The Thames island is an impossible dream.
The land to the north of Heathrow is suprisingly empty actually.