Originally Posted by
southlondonphil
There's a watermarked version on stock photo agency Alamy
here which dates it as 1938, though per
Wikpedia British Airways had moved to Croydon by February 1937 due to repeated water-logging of the runway.
EDIT:
This version from the Britain from Above website is captioned 1937
That would be because before LGW was built, the area was known as Crawley marsh.