Ideal location for Heathrow is at Heathrow.
Cities which relocate their airports to new facilities much further out from the city area have a long history of failure to varying degrees, and invariably the shorter distance operators refuse to move.
Montreal - Mirabel airport a complete fiasco, eventually closed and everyone moved back to the old convenient facility.
Dallas - New DFW airport, established airlines moved over from the old Love Field, new competitor (Southwest Airlines) moved in to the old facility, customers substantially stayed there as well, or shifted back as Southwest expanded.
Paris - Charles De Gaulle led to split operations between CDG and Orly, domestic traffic remained predomnantly at Orly.
Tokyo - new airport required International traffic to mve, domestic operations refused to move, proximity to city far more important to them than connections, has wrecked connecting traffic from Japanese provincial points. When new Osaka opened later, exactly the same thing happened there.
I could go on all night with examples.
Face it, a major airport for a metropolitan area needs to be convenient for its customers. Few, and increasingly fewer as time passes, are going to/from the central area which the so-called high-speed ground transport is proposed to serve. How great for all the regular execs who use Heathrow and live in High Wycombe, or Guildford, or Chiswick, will find it if half their flights get moved to the Thames Estuary, and the other half stay at Heathrow. Will lead to a real downturn in the London economy compared to Frankfurt, Amsterdam, etc.