There is an Indian company who used to ship muzzleloading rifles and shotguns to the UK: http://www.curioushouse.com/home.htm I know that they made a very nice 12 bore ML shotgun. Maybe you could send them an e-mail and check. Pradeep Singh Parihar is the man to ask and he is usually very friendly and nice to deal with. Tell him HI from me if you remember.

Good hunting!