Berenice, the officer is wearing a mourning band. This could either mean:
- the death of one of his family, for example Lord Roberts wore one for some time when his son, Freddy, died after Colenso.
- respect for the passing of Queen Victoria [22 January 1901]. Many officers wore mourning bands for the Queen; I don't know exactly for how long they wore them.
I think the latter explanation is more likely in this case, as it was commonly done. If so, the photo would be dated to the early part of 1901, which would fit with the men being mounted infantry - their numbers grew and grew during the South African War as it became clear that it was necessary to fight Republican forces in the same manner in which they fought.
It is worth remembering that in theory mounted infantry moved on horseback and fought dismounted, like the Boers, and unlike Cavalry who remained mounted in combat.
The past is not dead. In fact, it's not even past.