You can transfer an antique muzzleloader onto your gun license. If it is smoothbore it goes on the SGC, if it is rifled it goes on the FAC.
Put it on the respective certificate and inform the police. Send them a copy of the details the gun (they may want to know its provenance), and send them a photostat copy of your license where it has been put on.
Put "from my collection" where it would normally be filled in by a dealer if you bought a certificated gun.

When (if) you decide not to use it anymore it can revert back to Section 52 as antique/curio.
Do the same and inform the police of what you have done and show it cancelled on the certificate.

Some police forces will tell you that once a gun (antique) is put on a license it cannot revert back to antique status. This is not true.
It does not state this anywhere in he firearms act.