hmmm this may be a hard one to crack. You guys sure put some odd angle pics up for ID.
OK looking at where the mudguard meets the running board I would exclude this being a Chev.
So my guess is either a 1940 or 41 Ford or since it is British (I assume RA = Royal Artillery) it may be a Fordson.
sorry closest I can get with a pic that really just says...."Hey I am a truck with a gun on my back"
Cheers
Cliff