I don’t know enough about the history of our spelling to be able to explain with certainty the oddity of having the letter o correspond to the pronunciation uː in the words move (and remove) and prove (approve, improve, reprove), but I assume we have to blame those Norman clerks who didn’t like to write u next to the then identical v. Awareness of the etymology may have played a part, too, given that these verbs come from Latin movēre, probāre.
Despite the spelling–pronunciation anomaly, there is no tendency at all to regularize the position by adopting a spelling pronunciation. No one says məʊv instead of muːv, or prəʊv instead of pruːv.
But people in Britain do quite often say ˈprəʊvən rather than the expected ˈpruːvən for proven. This is one of the two possible past participles of prove, alongside proved. To some extent it has taken on a life of its own as a prenominal adjective (as when we say “a proven track record”, though never “a proved track record”).
The variant ˈprəʊvən seems to be quite recent. It is not mentioned in EPD prior to Roach’s editorship. Unless someone can prove the contrary, I shall tentatively claim that LPD was the first dictionary to include it, just twenty years ago.
According to OBGP, it is particularly associated with the Scottish legal verdict not proven. Is this true, and if so, why?