Why subs? Many folks I know use them because of a number of important reasons.
1. Subs are easier to obtain than ANY BP - with so many different variants of BP around, and the best only available at one or two locations in the entire UK. The HAZMAT carriage cost added to the base price makes it costy, to say the least. What do you do if you live in the South West, and Kranks is 250 miles away?
2. Ease of storage - no fancy Home Office-approved box needed - although some of us, me included, store our propellant in a large wooden ammunition box anyhow.
3. No extra license is needed for subs.
tac