FYI, most leisure-type long black powder firearms in this country tend to fall below the legal requirement for muzzle velocity or muzzle energy, or both. Hunting or taking game with a black powder handgun is prohibited. However, muzzleloading shotguns are still pretty popular, both percussion and flintlock, and for live game shooting, too. There are also club-style competitions in clay shooting for this category of guns right up to international level, as, indeed, there are for ALL types of black powder arms.
You might take a peek at Youtube - tac's guns, and see some of the BP stuff that we shoot at our club in the East Midlands.
Anyhow, Sussex, East and West, seems to well-equipped with gun clubs, according to Google. Remember that you MUST make your initial contact with the club secretary - just turning up on spec is NOT a good way to start your club life.
tac