Muzzle loading and black powder is alive and well in UK.
The people to contact about making a M/L from scratch are http://www.mlagb.com/
The Muzzle Loaders Association of Great Britain. Their HQ is at Wedgenock Shooting Ground near Warwick but they have groups up and down the land. One of their leading lights (might be the Chairman) is Peter Starley a business colleague of mine from way back. Pete is a Registered Firearms Dealer and has his own part time business supplying components plus buying and selling guns. Find him here ...... http://www.black-powder.co.uk/order.htm
Speak to Pete, he's a sensible bloke, and see what his advice is. At some stage your local Firearms Liason Officer will probably be involved. These guys are usually civilians and their degree of helpfulness varies, so my advice to get your story straight beforehand. The licensing procedure isn't all that tedious; a mate of mine has black powder guns and shoots game with them a couple of times a year.
To make a gun for your own use you do not need to get involved in Proof. That only applies to guns that are to be sold or given to a third party.
I've got a set of sketches / drawings for a repro Harpers Ferry black powder pistol. PM me with your postal addy if you'd like a copy. It'll be my project before long.