So that's what they were for

As a fan of The Young Ones I've always assumed that the flashes of found footage was a send up of the notion of subliminal messaging or just goofy gag.

That's from "Bambi", a.k.a. - the University Challenge episode.

Now I learn that these images were part of a running gag that was meant to have a payoff at the end of the show but that it was nixed by bureaucrats.

So the flash frames were never intended to be just a random piece of nonsense. They were meant as a running gag throughout the six episodes, which cumulated in a punchline...All of which meant that when “Summer Holiday” was broadcast on the 19th June 1984, it was shorn of its intended flash frame, and the punchline to the whole joke.