Oh, come on David. Everybody knows the answer to that question.
It must be simply because our coal deposits ran dry, fortunately just as our need for coal evaporated...
"Coal in the UK - 2012
The UK consumed 64.1 million tonnes of coal in 2012, including 54.9 million tonnes in power stations.
Coal imports to the UK were 44.8 million tonnes, a large increase (+37.7%) on the previous year's amount, mainly as a result of a dramatic increase in electricity generated from coal. Indigenous production was 9.9% less than the previous year at 16.8 million tonnes. (Over the year, 3.0 million tonnes was lifted from stock, compared to 0.8 million tonnes in 2011.)
Coal-fired power stations provided 41% of the UK's electricity (gas 26%, nuclear 20%, others (including renewables) 13%).
(Source - DECC statistics)
UK Coal Imports
Coal is therefore a fundamental component of the UK's energy mix, and around three quarters of the nation's requirements are imported.
CoalImp - the Association of Coal Importers - believes that imported coal complements indigenous supply as a secure and essential part of the UK energy balance. Coal resources are plentiful, distributed more evenly around the globe than oil or gas, and coal is capable of being transported and stored safely."
http://www.coalimp.org.uk/3.html