When I trained for my IR in the UK (Bournemouth = EGHH?) there was a flight school that didn't use the registration numbers but their own call sign [namexxx]. I haven't seen an ICAO flight plan in years but I do remember that you can enter your desired call sign instead of the a/c registration. In the remarks section you enter the regular a/c reg.
I almost forgot, during my IR exam, my call sign was EXAM57 or so.
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P.S. HD, what were the call signs for BOAC and BEA? speedbird?