The British Isles is by no means a uniform geological entity. Coal measures are found more-or-less in places which alternated between shallow seas and swampy forest when our fair islands baked under a tropical sun around 300mya.
Looking now at the UK, the Scottish Highlands are part of a vast mountain belt running from northern Norway to the Applalachians, which predates the opening of the Atlantic Ocean, and just happened to have got stuck onto England instead of Newfoundland. The Lake District and Wales owe their existence to the tectonic upheavals which tore apart the supercontinent of Laurentia, whilst granitic Devon, Cornwall and the Scillies are but worn-down stumps of the vast Hercynian mountains which were thrown up by even more ancient spasms. And just for good measure, the ubiquitous Old Red Sandstone delineates a vast, iron-stained Devonian desert the size of Europe.
In short, Coal was formed under fairly fussy conditions which only persisted for a relatively small timescale, and fortunately for us, stuck around long enough and was accessible enough to be of the greatest utility to us in the Industrial Revolution.
MARK