According to "The Standard Directory Of Proofmarks" those are from the Birmingham proof house, but may not be the exact location of the barrelmaker. Barrelmakers were separate guilds. There were hefty fines and even confiscation of guns without proper (or faked) proofmarks in England. Barrels imported were required to be proofed as well, marked with various things - gauge, caliber, black powder and/or nitro proof, etc.
Some markings referenced "balls to the pound" rather than bore diameter in inches, etc, etc,
The underside of the barrel may well bear additional info. Location of proofmarke on this piece would permit verification of barrel proof without disassembly. Nice gun, by the way. England produced muzzleloading shotguns even into the 20th century. Military arms varied in proofmarks according to the book. This gun is a sporting gun, not military.