Well there was a different structure within the ATA, especially by the end. The ATA were effectively part of BOAC, almost a military arm of the civilian airline which is why it took on the ranks. As the war progressed the ATA recruited a group of WAAFS and also newly trained RAF pilots, where previously the flying personnel were all civilians considered unfit for front line military service through age, gender or disability (saying that, there was at least one one-armed pilot, numerous very compitent females, and very good over-the-age pilots).
The ATA set up a training cadre, which is where the Cadet rank featured, and it began to be run more along the lines of a military organisation even though its personnel were still mostly civilians, whereas BOAC remained a true civilian service on war work. In addition, Im not aware of BOAC taking on and training aircrew like ATA did by the end of the war. ATA had its own training school for the purpose.
In addition to its own staff, ATA also employed numerous ATC cadets, who took on minor 'flight engineer' roles such as hand cranking the undercarriage on twin-engined types and the like. These were all drawn from nearby units rather than ATA having their own groups of ATC cadets. They began by doing minor jobs in the hanger and the flights began as a sort of thank you.
I'll see what I have on BOAC in the way of photos, but for now here are some IWM ones, including a shot of a two and a half ringer...