Hi
Everything about this photo is compatible with the Royal Highlanders (Black Watch). (1) The cluster of short black tails on the sporran resembles only the 5 tails worn by the Black Watch; (2) the feather 'hackle' on the bonnet is not white but dark, so can only match the red hackle of the Black Watch (3) dark hose tops must be the red and black pattern worn by the Black Watch (4) the blurred collar badge is the right shape for the Black Watch.
So this is either the uniform of the Black Watch, or a volunteer, militia or colonial uniform closely modelled on the Black Watch.
The rifle is a Martini, os if he's in the UK the photo is probably before the mid 1890s - if its a colonial unit based on the Black Watch it could be as late as 1900.
T