I'll add a bit more on here frank.
If you want to put it together as a firer, first get a variation on your FAC if you want to do it with a rifled barrel. If you have a SGC you are not limited by number so you could put it together as a smoothbore then have it proof tested.
I had a similar question because I want to build a wheellock. It can be done if you get the slot on your license first. you then build it then write it on as aquired then submit it for proof testing.
As I wrote earlier I'd do it for a smaller bore than original say .577 or even .44
Could then be a nice target gun.
Look here for a barrel.
http://www.trackofthewolf.com/(S(scb....aspx?catID=14
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane