Brian and Karl remember a form of English that's all arsey-varsey: "slang used in Britain by actors, circus and fairground showmen, merchant navy sailors, criminals, prostitutes, and the gay subculture."
I'd hazard that it's possible to follow the gist of things for native English speakers—but Amelia Bee explains the plot in YouTube Comments:
Here's a basic summary for those who don't understand the slang: The setting is London in the 60's, when being gay was still illegal. Two gay men are on a bench. One comments that he doesn't like the book Clockwork Orange. Using coded language they check to see that one another is gay before letting their guard down and speaking frankly, ogling other men as they pass by, etc.
They gossip about a promiscuous mutual acquaintance that got thrown in prison after getting caught having sex with men. The one on the left then laments that he nearly got locked up himself once, after the cops came knocking right as he finished going down on a guy, but narrowly escaped by telling them there was a "poof" inside and ran as they arrested his lover. The one on the right is rightfully disgusted by this revelation and leaves.
