On the one hand they do, as they certainly don't mean Cockney or Essex or Manchester.

But on the other hand they don't, because true RP is generally far too pretentious. Even in movies/TV, you see it mainly only in villains and historical dramas, or for comedic effect.

Most people are generally referring to a kind of middle-class London accent. Like Jude Law for example. Neither "posh" nor "regional" nor "lower class" nor "multicultural" (quotes for irony where necessary, obviously London is a region). I'm not sure what the name for that is though?