The design of the South Kensington Envelope was copied and reused with modifications for many other purposes. Furniss' caricature had also be so treated in 1971 for a Mail Strike.
One of them was the cover below, 'updating' it to include airmal service from the Wright Brothers to the Concorde. Neither of which really had much use to the postal services.
There's no stamp show or Concorde event tying the date of 2nd July 1978 - so it's a bit of a mystery why so dated.