S. Instone & Company Limited, a shipping company set up by Sir Samuel Instone, had operated a private service from Cardiff via London (Hounslow Heath Aerodrome) to Paris - Le Bourget Airport since 1919.
There is a history of the air line: Early Birds - Air Transport Memories 1919-1924 by Alfred Instone (published by Western Mail & Echo, Cardiff, 1938) containing a number of photographs.