Could be worthwhile considering pyrodex as a propellant (no need for a black powder licence) and later on if it lives up to expectations go to the expense and "hoop jumping" of a black powder licence.

Regarding loads I would suggest contacting the Muzzle Loaders Association of Great Britain.

Make sure that whatever recommendations you get are for single round ball / smoothbore and not "shotgun" load, and start light and work up.

There is also the can of worms of licencing if your gun is on a shotgun cert!

Cheers