If you're heading towards Fenland Rural Sports, it might be worth stopping off in Chatteris on the way...
Jon Francis at CPG Design has a few BP pistols on his current list.... you'll need to call him before going as he works from his house, and isn't always there...
https://www.cpgdesign.co.uk/firearms
If it's Pyrodex you are after, remember that it's P grade that you'll want. Else Triple 7 is good, and slightly more 'vigorous'
For example -
30gr of 3Fg black powder = 27gr of Pyrodex =25gr of Triple 7. More or less.
No documentation is required to buy any of the subs.