Be aware there are 2 versions of 8mm Mauser. The one you are describing is the old version with a .311 bore and .318 groove diameters. The more modern and more common version, and the one you're most likely to get in a take off barrel is .318 bore, and .323 groove. The twist sounds too fast for black powder and lead.
IIRC Kranks used to sell a muzzle loader called a "Squirrel Rifle" that was .32 calibre, so the bore size should be at least workable.
The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.