One big problem - LHR is our NATIONAL airport, it happens to be close to London.
Manston is in the worst possible place for almost all of the nation in the bottom right hand corner
The best place for a Heathrow replacement would be north and west of the current site, somewhere near BHX with an HS2 station would be good. Accessible from the M5, M40 and HS2 and the journey time by HS2 to central london would be faster than the current tube line from LHR.