Paul Terry wrote:

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.

Heathrow, like nearly every main airport in the world, is upwind of the city
it serves, in order to maximise visibility at the airport.