In addition,
The book "Blockade Runner", by Lars Nilsen and Leif Sandberg offers quite a bit of info on the communications with the Swedish operation running in the opposite direction....
A prearranged sequence of telegrams was sent with callsign, registration, crew and passengers on board, prior to departure.
Minimal VHF was used, if at all, to prevent the Germans realising what they were up to. Bromma at this time was full of spies.
Mention is made that some diversions into Dyce, Kinloss and Leuchars occurred, resulting in SOP's being devised between the Air ministry, RAF, BOAC and Swedish.
Sorry this doesn't answer the original poster's question re BOAC callsigns but the info into BOAC ops may be out there somewhere?