The market outside the UK, I predict, would be far more likely to take you up on your plans. A number of European countries count BP single-shot firearms as non-guns. As pointed out earlier, BP arms are not even 'firearms' in the US of A. The crunch here in UK is that, as noted, any replica MADE after 1938/9 is automatically a live firearm of one kind or the other [Section 1 - rifled, section 2, smooth] but ANY pistol, even if it is a smoothbore, is counted as the same as a rifled gun of any kind. A pair = TWO Section 1 firearms - that is, sad to even admit it, one more than many gun owners in the UK actually own.
In the USA where almost any Joe can amble into a gun store and walk out laden to the point of staggering, with any BP gun, the sales will be different. Notwithstanding Mr Hallam's comment about parsimoniousness, I think you'll find them being ripped out of your hands by the many thousands of collectors of fine guns stateside.