I beleive there was a proposal a long time ago to place a massive airport near the WCML but NIMBYs ruined it?
TBH, I can't really see any better places for an airport than Heathrow or Gatwick without massive inferstructure projects to improve access, anything on the kent side of London is in the wrong place as it will prove access from the rest of the UK needed to go through or round London, and thats a major obsticle.
Expansion of Heathrow might be unpopular, but it may well be the only viable option if you consider access requirements, improvments in access to Heathrow can be provided as part of HS2, Crossrail, M25 Widening, Airtrack etc.
Where as if there is a link road and HS shuttle from say St Pancras to the Isle of Grain airport, everyone would need to change in London, potentially (as is the case with Heathrow now) Except heathrow is near other transport links.
With a more logical layout of terminals being developed now, the limiting factor at LHR is runways, there is space if you sacrifice Terminal 4 and the Cargo terminal (or a large part of it) for an additional runway, with just enough clearance between them (Unlike Gatwick). It would need to be shorter and right up to the permimiter fence though.
Realistic expansion could also be acceived by pushing across Bath Road and moving the maintenance area to somwhere else, replacing that with a terminal (for terminal 4 and the cargo terminal's loss) or pushing further south to the east of Stanwell, moving terminal or maintenance space there to free up space on the current site for a runway on the southern side where the cargo terminal currently sits.
Any additional runway within the current site wouldn't be able to take anything larger than a 767 though, so it would improve reigonal links through Heathrow allowing the current reigonal slots to be used for larger aircraft.
Either way, I'm more in favour of Heathrow Expansion and better intergration with other airport to share capacity than building a new one on the wrong side of London.