In Scotland, there was plenty of slate near the surface to quarry it. I don't recall hearing of any slate mines.
I understand they recently reopened one of the quarries to get slate for some of the older listed buildings in Edinburgh. The building department vetoed the use of cheap slate from the continent that you could shoot peas through!
Billy (Son of a slater who made me carry tons of the stuff up ladders in a hod when I was a youth).