The so-called circle of confusion (pace Toole) is a hi-fi listener's invention. It's not really a thing in the recording world. Very few engineers work only in one place. They carry in their heads a kind of consensus database - they know Studio A in London sounds a bit lean, Studio B in New York a bit warm, Studio C is Los Angeles a bit bright, and so on. (I used to carry a reel of 1/4" tape, later a CD, to run a kind of mental calibration check before I settled in.) They adjust on the fly. The idea that mixing on B&Ws means instant worldwide disaster just isn't true. Give those folks some credit. They have a thousand times more worthwhile experience than any domestic consumer.
Are you talking about mixing studios or mastering, because they are clearly not the same thing. And the wide availability of horrible sounding recordings across many years and all genres proves your theory to be false. How much credit should I give to an industry that is now on a 20-year tear of creating recordings with less than 8 dB of dynamic range?
There are differences in mixing/mastering rooms sure, not to mention individual tastes of the mastering engineer, which makes it all the more important to use properly engineered speakers.
This is silly too, and unfair to the BBC. First, BBC speaker development was a long time ago, in an entirely mono world, for entirely private and specific professional purposes - mostly live location mixing of classical music and talk, in outside broadcast vans. Those purposes had nothing at all to do with audiophiles' preferences more than 50 years later.
Outside broadcast vans in the 1960s and 70s were nothing like the huge luxurious semi-trailers you see now. They were very small and very narrow box vans. You sat with your forehead almost touching the grille cloth. The "dip" was requested by the users, partly to quieten an evident Bextrene "quack", but mostly to push the sound backward to a comfortable perspective. As such, the (lone, mono) speaker did its job very well.
It isn't silly, you proved my point with your story. We are being saddled with an intentional dip to mimic poor conditions from 60 years ago. As I said, insane.