On this link it only states you cannot use a bow or crossbow for hunting. It does not mention muzzleloaders.

https://www.gov.uk/hunting

I'm sure I have seen it somewhere that you cannot use muzzle loading rifles for small game. This may be be down to the Chief Constable rather than what it states in law.

Would your FEO allow muzzleloaders?

Bodies like the Forestry Commission may not allow it so some of this may also be down to whoever has the say over the land where shooting takes place.

There are far too many grey areas in the UK firearms laws. Something needs to be sorted out!

I'll keep looking as I would like to know the law on this.