Thanks for the link, Corum. I noticed that, although a small number of the googled sites referred to the British upper-class accent, most referred to how people from different Scandinavian countries sound to each other, some to how Americans sound and a couple to how speakers of different French dialects in Quebec sound.
So, if one wishes to use an idiomatic expression for the way in which upper-class English people speak, a plum in one's mouth is probably more widely recognised than a hot potato.