Now that is a thing of beauty.
It is a British S.B.M.L. mainly for naval use. The No. on the trunnion is its serial number, it was cast at the major foundry of Carron (the foundry that made Carronades famous). Cast in 1812, of Blomefield pattern. Blomefields designs dominated the British artillery scene from the late 1700s to the early 1800s - approx 50 years.
It is probably a 24 or 32 pounder, a bore size would ascertain that. There will be a GRIII raised royal cypher on the 2nd reinforce. Also will be marked weight, several other marks & sighting lines etc including the broad arrow.
It's mounted on an original cast iron garrison carriage, these are also marked with type & weight etc, a deep broad arrow even being visible in your pic. The C.I. carriages were used mainly to spare timber ones from sufering standing decades in the weather - Apparently if battle was likely the guns were swapped to the timber ones as they did not shatter so when struck as did the C. iron ones. The cast iron carriage is later than the barrel as I understand that they were a later innovation - can't remember when introduced though. Also it was found that the heavier guns didn't need trunnion covers (cap squares) for garrison use.
A lovely gun, do you have any pics of the vent field & top surfaces of the barrel?
Adrian