I do not think this is a British gun, despite where it currently resides. The proof marks were probably added later in life, after the barrel was shortened in Great Britain. I think it is an American rifle, taken to England where it was "Englishized" by getting rid of the excess barrel, then proofing and marking as such. The British gunsmiths usually did such a good job when reworking guns, that it is sometimes difficult to realize what the original configuration was.
I think the gun originally had four pipes, thus the probable loss of 10 inches or so. This gun has distinctive details of two (state of) Kentucky gunmakers. I would suggest it could have been made by either Madison Hubbard of Nelson Co., Ky, or William Lutes of Nelson Co., KY and later MO. The odd guard is similar to one on a signed Hubbard rifle, particularly the heavy rear spur, and the patchbox finial outline is a dead ringer for an 1827-1830 box on a gun I have. Kentucky guns also had longer barrels, 44 inches plus at this time, which the British would probably detest and quickly get rid of. While this doesn't prove anything, there is a good possibility the gun is from one of the two men above. I'd still like to see the toe plate and the tang to see if either one supports this possible conclusion. Shelby Gallien