Agree, the proofs mark will tell the tale. Value will be based upon a lot of things when it comes to SxS's. Is it a sidelock, a boxlock, or a percussion gun; are the barrels still in proof; are the barrels on the face; are the ribs still tight; etc. Joseph Bourne was a Birmingham maker and like many had a store front in London to claim they produced "London" guns. I am The proof marks will tell you whether actually Birmingham or London (both cities produced wonderful guns).
You say it is a black powder gun - cartridge or muzzle loader? If a cartridge gun, it is almost certainly proofed for a moderate 1 1/8 ounce max load and 2 1/2 inch shells. If black powder, then max 800psi loads. I would not use 2 3/4 inch rounds in it and never heavy 1 1/8 - 1 1/4 ounce loads.
Also, Belgian makers often produced copies of English guns though usually of much poorer quality. Richards was often copied and a friend has a "Webly Scott" proofed in Liege rather than a "Webley and Scott" proofed in Birmingham. Bottom line it could be a fabulous gun or a clunker and anywhere in between. Just need to do a bit more dective/evaluation work.