Pre-war, or early war British; the cap badge is Royal Artillery. (The white lanyard also points to the RA) The collar badges are not the usual grenade type worn by the Regular Army, so I would suggest that they are the collar badges of a converted Yeomanry regiment (converted to a Royal Artillery Field or Anti Aircraft Regiment). I will stick my neck out and suggest - as it's difficult to make out definitively - that he is formerly of the Hertfordshire Yeomanry and therefore possibly 86 or 135 Field Regiments, RA or 79th Heavy Anti Aircraft Regiment.
