IF this is a .455, not maybe a .44 Special or .44/40 , it's still an early Triple Lock. I think most had a gold colored medallion at the top of the grip plates. Ths gun may be too early for that.
Is the barrel marked as to caliber?
This is NOT a MK II, a name usually reserved for the Second Model Hand Ejector that replaced this more expensive gun. See that extractor rod shroud?
The British felt it made the gun too heavy and maybe more prone to jam in a muddy trench. It also added to cost.
In those days, officers bought their own sidearms, so a name on one isn't rare. This one may considerably pre-date WWI and may have been obtained on the regular commercial market by the owner. The lack of those brass (?) medallions on the grips makes me think that's the case.
The gun seems not to have been carried much at all. It is probably quite valuable. I'll let the specialist collectors tell you current prices, and they aren't low! The Triple Lock is regarded by many as the finest-made revolver ever produced by S&W and any example in this condition is a valuable gun.
How someone like you who doesn't even know what this gun is can acquire items this impressive just baffles me.

It is a rare prize for both S&W collectors and for British Martial collectors. You are very fortunate to have it, if it is your gun.
I think you need to quickly buy a basic reference book on guns. Apart from specialist S&W titles, the Triple Lock and other basic models are well covered in the late Elmer Keith's, "The Sixgun" and in Geoffrey Boothroyd's, "The Handgun."
How did you encounter it? I hope you bought a lottery ticket the same day. Maybe you're on a run of luck!