The sequence is a sort of set piece, presumably based on historical fact. But it seems, in the end, to be little more than an elaborate red herring. Leigh doesn't appear to be criticizing Gilbert's directorial style, so diametrically opposed to his own, nor is there any reason why he should. The scene simply sits there, on its own, a little enigmatic, but not suggestive enough to justify its length and detail. It's all very tantalizing and, in the end, dissatisfying, because little of it is developed, worked out.
Every significant piece is an argument for something.
Because one has to assume that Leigh chose his subject matter with some purpose in mind. Perhaps the film is less about the two artists per se and more about the working out of a possible approach toward their art, and, by analogy, the problem of the contemporary audience's attitude toward Leigh's art and its expectations of him. If this is the unconscious message, then it's something of a problem. There's a potential danger, I think, in doing difficult, demanding work and then offering what could be interpreted as a half-apology for it. Remarkable as Gilbert and Sullivan no doubt were, there still seems to be some kind of disproportion between the care lavished on the subject and the latter's relative slightness, so that one almost gets the odd feeling of a surplus of artistry accumulating in the film, artistry which, at times, doesn't have enough substance to occupy itself with and eddies about on the screen.