There was a force called the Royal Defence Force which operated in the Great War. The RDC were troops who were used in a variety of home postings where guards were needed. They staffed the POW camps, guarded the docks, munitions dumps and other vulnerable points, as well as the designated Special Military Areas. It was initially formed by converting the (Home Service) Garrison battalions of line infantry regiments. Garrison battalions were composed of soldiers either too old or medically unfit for active front-line service; the Home Service status indicated they were unable to be transferred overseas.
Your man may have been part of that force.
jds1949