I've been researching the 1914 Royal Engineers motorcycle despatch riders for some time now, in particular the members of 5th Signals Company to which Willie Watson (who wrote Adventures of a Despatch Rider) belonged.
All the despatch riders were volunteers who joined the Army on the outbreak of war. They were anxious to get to France because the rumour was that the war would be all over by Christmas. But they were not 'casuals' - all of them were properly recruited, promoted to Corporal, and were fully part of the military - a fact which humbled Watson and others who, as "amateurs" had great respect for the professional soldiers they served with. Many of them had relevant training (Officer Training Corps, etc) and were commissioned into other units within weeks of joining up.
I'm sure that your uncle will have a Medal Card (WO 372) and possibly, if he became an officer, a service record (WO 339) - try searching in The National Archives Discovery catalogue using these references. If in difficulty I'd be happy to look him up on Ancestry for you, just need his full name and date of birth.
