Well, it might be salutary to take a look at the Proms Archive, particularly the Glock years (specifically the 1960s), and note that every season, there was a Gilbert & Sullivan "pops" (to use an Americanism - sorry, can't help it) Prom. In addition, I'm reminded of this passage from a chapter in The Proms: A New History (p. 182):
It wasn't all Birtwistle, Maxwell Davies, Stockhausen, Boulez, and Nono during the Glock years."....Glock could be practical, too, as when he was asked by Richard Marriott, the Assistant Director of Sound Broadcasting, to ensure that Saturday evenings were good box office nights with programmes suitable for airing on the Light Programme. Far from dismissing this suggestion, and no doubt understanding it as a price for central Corporation support, Glock replied, 'You will be asking me for a tight-rope act, but it's the kind of challenge I enjoy.'"
If it's any consolation, the same sort of phenomenon is happening here with a few US opera companies, reaching out to stage musicals in addition to operas. Glimmerglass is staging The Music Man and Lost in the Stars this summer. Rumor has it that Lyric Opera of Chicago will be staging Oklahoma! down the line.
Getting back to the thread, I enjoyed hearing this Prom on iPlayer, although Anthony Andrews struck me as a bit OTT on the radio. Maybe he felt that the cavernous space of the RAH required that from him. But he can definitely sing more notes than Rex Harrison, which granted isn't saying much. Except for Alun Armstrong and Sian Phillips, the rest of the cast was unknown to me (but then, I am American), but everyone was good. It was interesting to hear a more "weary" interpretation from Jenny Galloway as Mrs. Pearce, compared to Mona Washbourne in the movie. One small touch in the dialogue was to make the American millionaire "Ezra D. Wannafeller", which was the name in Shaw's original Pygmalion, but which Lerner altered in the musical to "Ezra D. Wallingford".
But just from seeing still photographs on The Arts Desk blog, the performance must have been an amazing treat to experience live in the RAH. I'm not normally a big fan of musicals, but I would have been happy to Prom for this one had I had the opportunity.