Flintlock Bob - I think you'll find and about half of my total number of posts are given over to explaining UK gun laws per se, and a lesser amount of handgun law in Northern Ireland, none of which applies to us with muzzleloading firearms.
Basically, as Feltwad notes, here in ALL of UK [that's England, Northern Ireland, Scotland and Wales], you can have any kind of muzzleloading firearm, long or short, or handgun, up to and smoothbore up to 2" bore - that's a punt gun BTW, not a rifled gun.
In order to shoot your RIFLED muzzleloader of any kind, you MUST have a firearms Certificate, usually called an FAC, or just a 'ticket'.
To get THAT, you MUST be a full member of a gun club, and, having served six months provisional noob-ship to learn the ropes, you may apply for your FAC which authorises you to acquire and possess the gun of your choice. That would occupy a page here by itself.
As Feltwad notes, they cannot be used anywhere except on an authorised range, and in the UK there are NO un-authorised ranges.
Shotguns that are also muzzleloaders may be held on a Shotgun Certificate - any number - literally. Same for shotguns that hold up to three cartridges. More than that and they have to be on a FAC, and you must either be a practical shotgunner or have another so-called 'good reason' to possess it - like wildfowling.
Muzzleloading shotguns have their own competitions, and are much enjoyed by those who have them, but again if you use them they must be on your shotgun certificate.
You have to produce your FAC to buy percussion caps and sub-BP, or your black powder license [free, lasts five years] to buy BP.
ALL fireable replicas are counted as modern firearms, and are treated as such with regard to home security.
There is, of course, a LOT more to it than that, but this is not the place to go into more detail.
tac