One of the things that Thatcher did that I agree with was closing down the coal mines. To me, it's just simple free market economics. Cheap coal was being imported from huge open cast mines in other parts of the world. Deep-mined coal simply wasn't competitive.
When world coal reserves become depleted and carbon capture technology is perfected, there will be demand for the deep-coal and the mining industry will be back in full swing. Until that time, it should not be subsidised by other parts of the economy.
I know that Thatcher's policy towards the mining industry ruined many communities and perhaps it should have been done with a bit more compassion and support for the ex-miners, but closing the mines was the right thing to do.