Just to answer the "is BP made in the UK"... the answer is no..

The last dedicated gunpowder mill closed in 1954 in Roslin, just south of Edinburgh. It was run by ICI who closed it as the buildings were being damaged by subsidence caused by local mining, and the licence was withdrawn.

The incorporating mills were moved to Ardeer, ICI Nobel's main plant where they ran up until the 80s but were not used for making Black Powder. The mills were only used to manufacture meal powder for making safety fuze. As I understand it no pressing or corning took place at Ardeer and corned powder was imported from Troisdorf, Germany for resale as Curtis's and Harvey's and for pyrotechnic use..

I would agree with your advice that it is impossible to predict GP performance from the chemical makeup. There is wide variation based on milling time, density, grain size and moisture content...