You won't find US Style Kosher Salt in the UK, not even in Kosher Deli's. Unfortunately its just one of those things. I have asked at countless Kosher Deli's all around North Manchester, and even all my fellow Jewish friends mothers have never heard of it, it seems to be an American thing, I know, its hard to believe, but its just not easy to get here!
I use Maldon Sea Salt in flakes, thats the closet I find.