You might find a .22 LEM Spitzer mould if you ask in the Collectable Airguns section. This was a mould for casting solid bullet shaped airgun pellets circa 1970's-1980's. I've read of people using them for loading .22 Hornet cartridges so they might be ok for a BP revolver.
Alternatively, send an email to NAA and ask if they have an agent in UK (or Europe) where you can buy the bullets from. Sales contact from their website is kenny@naaminis.com
Good ideas.
You might also ask on the Muzzleloaders Association of Great Britain (MLAGB) Forum.
Jim
UBC's Police Pistol Manager
"Nasty, noisy things, revolvers, Count. Better stick to air-guns." Sherlock Holmes, The Adventure of the Mazarin Stone