This is the uniform of an English or Welsh infantry regiment that did not have a 'Royal' title (if it did, the collar and cuffs would be dark blue instead of white). This basic pattern was worn between 1881 and 1902, but based on the cap and the cut of the jacket, I would say this dates from approx 1895 to 1902.
At this time, regular (full time) soldiers were often photographed in their 'walking out' uniform, which was specifically for leaving barracks. This consisted of the cap (as seen here) and best tunic. This man is wearing his 'undress' frock in place of the tunic, which was common for the militia who did not have tunics so used the frock for full dress and working dress. Another feature suggesting he is a militiaman is the absence of a moustache, which was required for regular soldiers (although it may just be that he could not grow one!)
Therefore I agree with Shaun he's probably in the Militia. Note there were 2 part-time forces at this time - the (paid) militia and (unpaid) volunteers. William specifically looks like he is in the militia rather than the volunteer force.
(Note, from this photo alone you cannot be certain that he was not a regular - they had undress frocks, they just tended not to get studio photographs in them)
T
