Over 60% of WW1 service records were burnt during the WW2 blitz
Are you sure it was the 12th? Post WW1 only the Regular battalions of the 1st to 4th remained. The RF did not have RA battalions in peacetime but even if they had if he was TA in 1920 then he would have had a civilian job. The 12th ceased to exist during WW1 and was not around in 1920.
This is from the Long Long Trail WW1 website.
12th (Service) Battalion
Formed at Hounslow in September 1914 as part of K3 and came under command of 73rd Brigade, 24th Division.
Moved to Shoreham-by-Sea and later to Pirbright Camp in Surrey.
Sailed from Folkestone for Boulogne in France 1 September 1915.
17 October 1915 : transferred to 17th Brigade in same Division.
13 February 1918 : disbanded in France, with personnel dispersed to 1st, 10th and 11th Battalions.