I don't know. I am a very poor wannabe Richard Holmes.
A mounted unit. Boots trousers and bandolier.
Bandolier has single cartridges so guessing not clipped ammo ie smle time. The belt looks like a 1908 webbing belt with a revolver in closed webbing holster? With possible ammo pouch.
Slouch hat could of course be Anzac but maybe brit mounted infantry in South Africa late on. You've just posted British Army.
It wasn't intended as a definitive statement. For all I know he could be the Mafeking Cadet Corps duty Dog Walloper, and his shotgun got cropped out of the picture. Clearly he rode a horse, but you already stated more than I know when you identified his webbing/bandolier.
OK he was not an Artist, he was 3rd. Volunteer Service Company Norfolk Regiment (now I assume he was mounted infantry but Norfolk Regiment history is sparse or the picture is a bit a of Walter set up), he did 193 days in the Army, 150 overseas in South Africa 1902, he got QSAM with 3 bars: