Later than the Boer War (1889-1902), the uniform was introduced in 1908 but I suspect it is WWI vintage. The cap badge certainly does show the Prince of Wales feathers emblem and there are several WWI era regiments that incorporate the emblem within their badges.
He has whip under his arm (leather end) that together with the POWs feathers suggests a cavalryman, rather than the Royal Artillery or Army Service Corps.
There were two WWI era regular cavalry regiments that incorporated the POW’s feather emblem in their cap badges, the 3rd (Prince of Wales’s) Dragoon Guards and the 10th (Prince of Wales’s Own Royal) Hussars, but both have an addition scroll beneath the “Ich Dien” scroll that forms part of POW feathers emblem, so it’s neither of those; that leaves Yeomanry (part-time) Cavalry regiments.
There were 5 yeomanry cavalry regiments’ badges that incorporated the emblem, but like the regular regiments 3 have an extra scroll. Only two yeomanry regiments had the POWs feathers emblem on it own as their cap badge: The Royal Wiltshire Yeomanry (Prince of Wales’s Own) and The Denbighshire Yeomanry.
Enhancing the picture, it looks the shoulder titles, the 1st three letters look like RUY and the 4th possibly a C; as there is no units name of three or more words beginning with those letters, the U must be a W, making it the Royal Wiltshire Yeomanry Cavalry.