Uniform is unfortunately not much of an indicator - it changed very little from WW1 up until 1937, the webbing stayed the same, and aside from minor modifications (the jacket and perhaps trousers were tighter fitting with the 1922 pattern service dress, and the peak of the cap was slightly bigger with the 22 pattern SD cap) looked pretty much identical to WW1 soldiers. That said, by the general 'look' of the lorry, it seems to be rather glossy and very highly maintained (and VERY clean, ie no extra camouflage netting, petrol cans strewn about), so i'd plump for 1920's too