It’s a Volunteer Training Corps cap badge so definitely WW1. The VTC were effectively the Home Guard of the Great War and your man appears to be wearing the original VTC pattern uniform of a private. I’m sure the VTC aficionados will be able to expand on the exact badge and pin down a unit from its design and his shoulder titles.
Some basic info on the VTC here
https://en.m.wikipedia.org/wiki/Volu...Training_Corps