I do not agree on the Purple UK Slate variant! Due to the pattern which is visible on the other pics of the Vendor...
@DireStraights:
The only way you can confirm is a very high polished bevel, fast cutting abilities for a finisher and a very very sharp edge...
+ Visual Identifiers like green inclusions, smaller green points
The Purple slates mostly have the round or oval green blotches, which can shurely occur in all kinds of varieties...