It is certainly army, Second World War period, after 1942 as the front buttons are exposed. He is wearing a light coloured lanyard on his left shoulder. At that time, the Royal Artillery with whom a white lanyard is associated, wore theirs on the right shoulder so that, and the fact that his cap badge is not a Gunner cap badge. rules out the Royal Artillery. He is wearing two good conduct stripes on the lower left arm indicating 6 years good conduct (or undiscovered crime) so with the buttons, post war? Beneath the stripes is a trade badge of some sort, too much obscured to ident. No shoulder insignia to be seen.
The cap angle makes badge ident difficult. the RAVC wore a white lanyard and their badge had a similar shape, can't be certain though. It is the badge which is the main clue here.
Not too helpful I'm afraid
maxD