...If you don't believe me, you can hunt up a 1983 book called "Footlights: One Hundred Years Of Cambridge Comedy" which is the history of the Footlights amateur theatrical society at Cambridge- whose alumni have included since the 1950s most of the auteurs of post-music hall English comedy.
Footlights revues since 1960 have included the casts of Beyond The Fringe (Jonathan Miller, Dudley Moore, Peter Cook and Alan Bennett), Monty Python (all of them), The Goodies (Graeme Garden, Bill Oddie and Tim Brooke-Taylor), Alas Smith And Jones, and Douglas Adams (Hitch-Hiker's Guide To The Galaxy).
In 1981 the Footlights mounted an Edinburgh Fringe Festival show called The Cellar Tapes, whose cast included...Stephen Fry, Hugh Laurie, Emma Thompson, and Tony Slattery!
The Cellar Tapes show won the Fringe's Perrier Award and pretty much guaranteed everyone jobs for life in British TV and film. The scene of them at school doing an amateur theatrical show for the university dons is a reference to this, supposedly.
Of the film, despite an interesting concept, some good moments and a talented cast I found this film disjointed, emotionally cold, only rarely witty, and even faintly unbelievable at times --the scene where Thompson breaks down and cries is so reserved and smug it's like she can never really let go- which she never does in anything she's in anyway!
It's rather as if they want to thinly satirize themselves- but only thinly, as if they take themselves too seriously to open themselves to self-mockery. For a better take on this concept, I recommend the 1998 film "Final Cut" starring Jude Law which has the current mob of Britpack actors playing themselves in an improvised film-- often times for laughs.
It's amazing how far Branagh's star has fallen since 1992 when he was The Olivier People Actually Liked. I guess some people really do peak early- he did the movie of Henry V (and wrote his autobiography) when he was 26! Since then?....Anyone?...Bueller?