Does anyone know if UK law would prohibit a Semi-Automatic muzzle loading pistol?

There are a couple of design hurdles to make one work, but the basic principle would be a muzzle loading cylinder (as used in Nitro conversions or the Alfa-Proj etc) actuated on a mechanism similar to a Webley Fosbery.

Result would be either a revolver, or possibly a harmonica type block pre-loaded outside the pistol with the usual paraphernalia but with the ability to self-index.

I can't see any reason it would not be legal - other than because it's not been banned yet on account of no such firearm existing.

Any thoughts?