Antique blackpowder guns are available without any licensing. They have to be manufactured before 1939 AND muzzle loading OR obsolete ignition system (eg pin fire) OR obsolete calibre centre or rimfire. You cannot own ammunition, nor can you fire them, nor can you own one if you are a prohibited person.
Modern blackpowder guns need a shotgun certificate if smoothbore, 24″ minimum barrel and minimum overall fixed length of 40″
There are other modern blackpowder guns that will need a firearm certificate. If you intend to buy loose blackpowder for muzzle loading or loading of cartridges then you need an explosives certificate. Any blackpowder substitute does not require any licensing. Shot, bullets, unprimed brass, primers for non-brass cartridges nor percussion caps for muzzle loaders require a certificate.