Air Stewards were created in the 1960s on the introduction of the VC10 because there were not enough AQM/ALM available. At the time and until 1997 the criteria to become one was to be a substantive corporal or to be recommended for promotion, when an Air Steward was promoted to the rank of SAC Special Acting Corporal (Paid)
On completing a flying tour a steward would revert to being an SAC, unless they had been promoted to the substantive rank of Corporal.
I was also told that during the 1st Gulf War due to the numbers of passengers required in the middle east theatre of operations, C130 Hercules transports were often used and again, because there were not enough ALMs available, some Air Stewards were sent to Lyneham and after training worked on the Hercules for passenger safety reasons.
I did two tours on 10 Sqn, the first as Acting Corporal, and I was promoted just before the end of my tour. I returned four years later in the substantive rank and completed another three years before taking redundancy. From that point the trade changed as all stewards were employed on flying duties from day one and the rank structure also changed, SAC stewards stayed in their rank, but corporals who previously would have to return to ground duties on promotion to Sgt now could continue flying. The ALM role changed on large transport aircraft, so the whole cabin was run by stewards (cabin crew)