I believe that is what was referred to as a long 12 ...QF 12 pounder (12cwt) .
It is indeed a 12-pdr and "long" in the parlance of the time, so I'll give you the "What is this?" part. It was actually called the 12-pdr 18-cwt QF MK I. It is not to be confused with the 12-pdr 8-cwt, which was a designed-for-the-purpose landing gun. It probably came from HMS
Terrible, but perhaps her sister
Powerful. It saw action during the Siege of Ladysmith, crewed by RN gunners.
Interestingly, Percy Scott, the skipper of HMS
Terrible at the time, was a prophet of improving naval gunnery and his efforts played a part in WW1 fire control. Besides that, he himself designed the gun carriage shown in the photo.
Anyway, we've got the "what" and "where from" parts of the question. We've got Ladysmith as a hint to the "where". The main outstanding issue is, "how is this commemorated?" And the answer to that will answer the "where" part, too.