They are wearing Batledress so after 1938. The peaked caps were a inter-war design and mostly obselete by 1940 but a few regts (Guards, RHA, Corps of Mil Police) retained them
They are names as Gdsm (Guardsman) so are either Scots, Coldstream or Irish Guards. Coldstrams had the largest cap star so my money is them.
The Tunics are the pre war pattern with covered buttons. Mid wr they were replaced with exposed button fronts. I would suggest you can date it from 1938-41. It looks like a recruit platoon photo in basic training so if you know when he enlisted that will confirm the year.