Hi Gareth
This is going to be difflicult. I know nothing about horses, but if this isn’t a mule it has got to be one of the ugliest horses ever. The Boer War ‘consumed’ hundreds of thousands of horse, so it would not surprise me if they had to use mules.
The rifle is a Lee-Metford sitting in a leather saddle bucket. The rider keeps one arm through the sling so when he dismounts in a hurry his rifle comes with him, rather than disappearing when the mule gets spooked. It had an eight round ‘magazine’ into which you feed single rounds one at a time after pulling back the bolt. It was not designed to be changed. It looks like a standard infantry weapon rather than the shorter carbine intended for mounted units. See -
http://www.diggerhistory.info/pages-weapons/303.htm.
The boots also look infantry rather than cavalry issue, so they could be the mounted infantry section of an infantry regiment, kitted out with what was available. The bandolier held fifty rounds, and he would have had another 100 or so in the saddle bags.
The Sergeant in the middle is wearing a medal ribbon. It could be for the Queen’s South Africa medal, which was available late on in the war. Any man arriving in South Africa after mid 1901 say, would not get his medal until after the war had finished. The variety of hats could also indicate that it late on in the war. Is the man on the right in the front the same as the top photograph? Again his hat has changed to what he could find. And the water bottle doesn’t look standard issue to the infantry either.
They all look clean shaven which was more likely in an infantry regiment than one of the dozens of irregular mounted units created just for the Boer War, but I am surprised they are not showing any sort of badge. The only other thing I can see is that a few of the stripes look removable for easy washing of the tunic, and that must be one of the first wrist watches in the British Army.
Ken