Were do you find it for £10 a sack just looking on Ebay and £22 is the cheapest for 25kg still not expensive I guess
Your agri merchant should have it.
Here's Mole Valley's.
The mineral content of rock salt is negligible
It's true that 96-98% of the lump is sodium chloride, which is why it is called salt
The Himalayan stuff is unrefined, comes straight to you having been hewn from the rock by hand, and that 2-4% includes around 84 different things.
Trace elements in drenches, licks (and your vitamin tablets) and so on are so called because there is a trace of them, but that trace can have a significant effect on health.
At the price, I cannot fathom why everyone doesn't at least try it and make their own decision about its efficacy for their stock on their land. I did, and would not now be without it in every field, year round.