The man sitting on the ground on our right appears to have a wound stripe on his left cuff. These were introduced in 1918. Also at least half are wearing the ribbon for what looks to be for one of the stars, probably the 1914-1915 Star. This was authorised in 1918 but I don't believe pieces of ribbon were issued until early 1919. So my guess that that it is a demob photograph from 1919.
The ribbon was the same for the 1914 Star, but there are too many survivors for them to be wearing that. They also look too young and too happy to be old sweats.
The cap badge looks to be Royal Artillery. If they are RA I believe they moved the lanyard to the right shoulder in 1920.
Ken