If I am interpreting his RAF service record correctly, Berry joined on 1.7.18 to a RAF Cadet Distribution Depot. It looks like he was sent to the Lond(?) C.D.D., 6 days later. Could that be why he might be wearing a London Regiment cap badge? He was then sent to the 10th Technical Training Wing the following day. Some months later, he was sent from the '6th C. Of Ws/Wg'(?) to the '8th S of A.' from which he transferred to G class Reserve on the 6.1.19. So, I suppose the photo taken the next day was a last chance to get one of himself in uniform.
I am not sure of the abbreviations. It would appear that the unit he was with when the photo was taken was the 8th S of A, whatever that was. That is, unless he joined the London Regiment the very next day, or as a Reserve unit?
Many thanks for everyone's help.
Chris
Edit: Regarding joining the next day, I don't think I was thinking straight. It would be unlikely that he would have uniform and sergeant stripes in a day!