xkcd

Except that the UK taxed all its locally produced coal so much that it was cheaper to import more dirty coal from China than use fresh, relatively clean coal excavated from Wales. If a power station wanted to use Chinese coal, they only had to pay import duty. But if they wanted to use cleaner Welsh coal they had to pay taxes on the extraction, refinement and then the same export tax they levied everywhere else. Welsh coal was some of the highest quality in the world, but it was exported and more toxic stuff burned locally because that was more profitable.