I came across the following information on a WW2 RHA board:
"A lot of the TA, RHA and RA, were still in Service Dress on the outbreak of war. In general this would consist of a peaked cap, service dress jacket, breeches or trousers, long puttees and ankle boots. Some had already changed to Battledress."
So, it does appear that he's wearing a TA/RHA uniform.