The black-out mask clearly points to WW2, and prior to 1945 by which time restrictions were being relaxed in the UK.
The paint is peeling on the Nissen hut and the sandbags have been there a while. The bike has clearly seen a bit of use.
My instinct puts the photo a few years into the war and as the rider is wearing his issue wellingtons, probably winter. Perhaps 1942 - 43 although the winter of 43-44 was a notably wet one which might tie in better with the wellies !