You are correct in spotting that all four of the National Regulations restricting lead shot refer at their main heading to a “prohibition on the use of cartridges containing lead shot”.
However, the relevant section of the regulations, whether in England, Scotland, Wales or Northern Ireland, states that “No person shall use lead shot for the purpose of shooting with a shotgun”.
Thus, the letter of the law applies both to breech loading and muzzle-loading guns alike.
Loose bismuth and ITM shot supplied for homeloading are suitable for muzzle-loading guns.
Is it legal to use a muzzle-loading gun with lead shot to shoot wildfowl