I think that the motorcycle is most likely a Douglas (from the handlebar shape and mudflap supports which are typical), possibly a model B. If you know different then please let me know.

The back of the photograph has the name (possibly) J F Perry. Any information then please get in touch.

The motor cycle I have identified as a P & M Military Model 3.5HP. The distinctive front forks and mud-guard contour, the cylinder pointing up and forward and the characteristic crank in the lower rear axle support make this fairly certain. Also my reference, Ansell, David (1985) 'Military Motor Cycles', states that, "....the P & M factory received a contract to supply the Royal Flying Corps during the First World War with their 3.5HP model...".