it would have been unusual for a Guardsman to commission into the Guards as the officers social status was very different. Saying that if he was a former public school boy who was conscripted then it did happen. I met an ex Guardsman who was the 2nd Battalion CO's batman with the Irish Guards in 30 Corps at Arnhem and that is exactly what happened to him. The 2nd Battalion Irish Guards were an Armoured Battalion equipped with tanks so a course at the Armour Training Centre at Bovington would have been apt.
From another Forum:
On the outbreak of the Second World War the Royal Military College was replaced by the Sandhurst Officer Cadet Training Unit of two wings. These became respectively 101 Royal Armoured Corps OCTU and 161 Infantry OCTU (RMC). A London TA unit, the Inns of Court Officers Training Corps, formed the RAC OCTU and the Royal Military College formed the Infantry OCTU, with the subtitle RMC. Officer cadets of 161 Infantry OCTU were allowed to wear the RMC cap badge, although they were in fact soldiers belonging to the various regiments into which they were called up for the duration of hostilities. In 1942 101 RAC OCTU amalgamated with 162 Reconnaissance Corps OCTU (formed from the Infantry Battalion of another London TA regiment, the Honourable Artillery Company) to form 100 RAC OCTU. At the same time, the Infantry OCTU moved to Mons Barracks at Aldershot, where it remained, retaining its RMC associations, until 1946. It then returned to Sandhurst, while the RAC OCTU moved to Bovington.