I remember being able to buy saltpetre AND sulphur at a chemist shop in Alton which is not that far from Winchester. No questions at all. But that was in the 1950s. I made my own charcoal!
I bought a small sack of ammonium nitrate in Rotorua (New Zealand) a few years ago. I wanted it for hydroponics but if I had mixed it with an oxygen acceptor such as sugar, flour or diesel fuel, I could have made quite a nice bang. I suspect that I would be asked why I want it now.
100 grams of saltpetre would make a piss poor bomb anyway. If you sat on it, and the other necessaries, it would blow your arse off but that is about all. R