This would be parade uniform and/or walking out dress pre 1914. The outbreak of war resulted in this uniform being withheld from distribution, with the guards only receiving their tunics in 1920. the Infantry never received sufficient tunics following the first world war to again adopt it as their parade or walking out dress.

the uniform would be Side cap with regimental cap badge, red tunic, Black trousers with a white belt. judging from the belt, the cuffs appear to be white, which would indicate an English or welsh, non royal Infantry regiment. However there was a significant amount of regimental variation and it's perfectly possible that they preferred to utilise white over Royal blue. Unfortunately the picture isn't clear enough for me to tell the cap badge, but it appears to be similar to this http://www.northeastmedals.co.uk/british_regiment/kings_own_badge.JPG

Judging from the uniform, this picture would have had to be taken pre-1914, of a private soldier from an English or welsh, infantry regiment, however due to the complex system of cap badges and regimental identities I can't be more specific.