This photo shows a group of Royal Observer Corps, including an officer.
The corps was founded in the 1920s, and didn't stand down until 1995. (One assumes they had sandwiches and a flask of tea).
I can't date the photo with any accuracy, but it certainly looks like it could be wartime: the only medal ribbons in view are Great War.
The variety of styles of wearing the beret is interesting, particularly "Old Bill" at front row left.
http://en.wikipedia.org/wiki/Royal_Observer_Corps