I have a photo of five men in uniform sitting in front of a field tent. One of the men is, I believe, my great grandfather. The photo is marked at the bottom as "56th Wessex Brigade. RA. Cooks."

I have no other information to go by. I'd like to try to track down a service record for my great grandfather (Harry/Henry Richardson, 1879-1938), but some basic searches in Ancestry and Find My Past have yielded nothing except his service in the Navy between 1898 and 1908 as a stoker, which I already knew about. I'm trying to find out if he was in this 56th Wessex Brigade at some point, either in WWI or sometime after.

From what I've read online, Royal Artillery brigades became regiments in 1938, so that would date this photo to pre 1938. But searching for "56th Wessex Brigade" online hasn't yielded anything definitive. I'm assuming this was a territorial unit, but I'm not sure. Does anyone know when the 56th Wessex Brigade of the RA was first formed, and where it was based? I'd like to pin down a possible time frame for the photo at least.
And if anyone can suggest how/where I could track down a service record, I'd be very grateful. Above all my aim is to confirm that my g.grandfather did indeed serve in the 56th Wessex Brigade, so that I can then confirm that it's him in the photograph.
Many thanks for any pointers.
Marcus