My fairly untutored eye suggests late nineteenth, early twentieth century, by the styling but that doesn't mean much, given governments' penchants for not changing things. I assume it is wool, which again to me suggest earlier - pre-1960s anyway - rather than later, due to cost. Not sure when denim became the fatigue material of choice but I would guess sometime after WWI. Are the buttons bone, metal, rubber or plastic?
Also, the brass eyelet on the trousers is odd - perhaps it came with a draw string waist rather than belt or braces? That might lend weight to the prison story, as I can't imagine any military fatigue uniform in the Commonwealth that wouldn't use braces or a belt. OTOH, the number of pockets seems odd for a prison uniform too - most have NO pockets, for obvious reasons. The long pocket on the right hip has to be for a specific object/purpose and may be a significant clue.
The broad arrow was used by the Canadian military, with a 'C' around it. Not sure what our prison service used but a broad arrow is quite possible. Might also have been used by other Empire/Commonwealth countries.
Interesting puzzle!
Peter