BUA had, at least partially, adopted flight number call signs in the late 60s - we used to get the series “British United 451,2,3,4,5,6,7 & 8” flying over the Channel Islands to & from the Canary Islands (operated by Britannias) between 1966 &
‘68’ , about. But, I can’t remember what callsign they used after the merger with Caledonian. Sorry.