As Callow points out, the nudity is an essential part of that scene (in a bath). While admitting that Nigel Planer's comic book about acting (I, An Actor) has an entry in the glossary which reads "Callow, verb, to expose one's genitals in the name of art," he writes that plenty of other actors have shown their all if the production requires it. The real problem with his appearing nude is likely to be the fact that, unlike Hamlet, Callow's body is not generally regarded as "the glass of fashion, and the mould of form."
No one complained at a very young, cute, Jude Law emerging naked from a bath and rubbing himself dry at the National (Les Parents Terribles) ten years ago, nor at the prolonged nude scene in the Royal Court's Outlying Islands last year when a personable young man got his kit off and began to make love to a similarly disrobed girl on a table top.
Acting can seem like showing off — or "shouting at night," in the memorable phrase — but it takes a certain courage, too. For a middle-aged man to remove his clothes and suffer the inevitable mental sizing-up of his assets requires more guts than most would have, and if there's a little too much gut on show, then all the more courage needed to display it in the name of art.