Originally Posted by
Beamr
I'll add Kar-Air.
Kar Air were a bit like the Austrian Air Transport entry in my list, just a "virtual airline" for marketing (and possibly licensing) purposes, as an offshoot of the national carrier, using their aircraft and crews for charters in marginal time. Sometimes with aircraft with their titles added, sometimes not.
BOAC did the same thing as "BOAC Limited" in the 1970s when charter carriers were picking off significant long-haul loads, with all-economy configured somewhat surplus 707s. They were mixed in operation with a couple of BOAC scheduled routes which used all-economy aircraft, particularly Toronto (which was also a mainstream charter destination for them).
BEA did things quite differently, and set up BEA Airtours (later British Airtours and later again Caledonian) as a fully independent operation at Gatwick - except that in the winter off-season one of their principal uses was being chartered back to the mainstream carrier at Heathrow, particularly when more than once there were major problems with the Trident fleet there. There were also holiday charters all along by the mainstream BEA/BA fleet from outstations like Newcastle or Edinburgh, with overnight charters to the Med and back of nightstopping aircraft.