In message , Tom
Anderson writes

It looks like the airfield was about half the size of Heathrow:

Don't forget that when Abercrombie was plotting and planning, what is
now Heathrow was a small airfield (150 acres) used mainly for aircraft
testing by the Fairey company. All of that was about to change very
quickly, of course, although Abercrombie would not have known.

In the end, I guess Heathrow offered more room than Fairlop for
expansion - but I also suspect that a site to the west, rather than
east, of London was preferred because flying was an extremely expensive
way to travel until the late 1960s, and Heathrow would be more
convenient than Fairlop for wealthy passengers needing to be chauffeured
to and from the west end, Kensington or the stockbroker belt.
--
Paul Terry