You need a source of phosphate, regardless if it is the K or Na salt. Pick up the enema and let us know what exactly is in it and we can guide you from there. Or else pick up the stuff that Anona referred to.
KNO3 is indeed saltpeter.
Currently I rely only on KNO3 for my potassium dosing. I don't add extra K. I suppose you could add a "pure" source of water softener salt, KCl, if you wanted to try another source of K. I am watching my plants to see if they show signs of deficiency before I add extra K.
As far as BP goes, don't worry about it. It stands for British Pharmacopeia, which means the product meets the standards referred to in this book (much the same as if you bought a chemical and it was followed by USP, NF, etc).