.577/.450 caliber Martini-Henry carbine, Mark II, made at the Royal Small Arms Factory, Enfield Lock. The two broad arrow marks indicate it was sold by the government to an individual. I would guess that it was then cut down for a child or a small person, following which someone else had the stock "restored" with a piece from another rifle.
The breechblock may have been removed for reasons of safety or (if in the UK) because of strict firearms laws. I would not rate very high the chances of finding one, but they are around.
The cartridge is a bottleneck number, originally with an iron base and wrapped brass body, later of drawn brass. Rifle and cartridge served at about the same time and were roughly equivalent to the American trapdoor Springfield and the .45-70 cartridge, although the Martini is a faster and stronger action than the trapdoor.