The complete electoral collapse (2012) only happened after a Democratic administration openly and gleefully pursued policies sure cause the collapse of most of the remains of the industry and then proceeded to gloat about it. Like, this isn't really a mystery everyone...
So Sibboleth, what happened in short, medium, and longer term electoral voting patterns in the UK (Midlands, Wales, etc...) after Thatcher destroyed the NUM after the massive Coal Miner Strike of '84-'85?
I'll have a go at answering it but it's a more complex question than it sounds:
In areas that were particularly heavily hit by the closure of heavy industry (including coal) that still haven't recovered are still solidly Labour but more by default than anything else. Labour's vote share has been dwindling for years, for example not so long ago Labour used to get 70+% in the Welsh Valleys constituencies, now they are under 50% in most of them but they still easily win these seats due to lack of a credible opposition. The Tories are still absolutely despised, UKIP's potential is limited due to it's links to the Tory right and prior enthusiasm for things like privatisation, the Lib Dems love of 'do-gooder' internationalism is an anathema to these kind of places and Plaid Cymru (in Wales) potential is limited due to their fixation with things like the Welsh language. Also voter turnout in these areas is terrible and interest in politics is at rock bottom.
But as the lesson from the SNP in Scotland shows these areas have fallen out of love with Labour and will happily abandon them should a suitable alternative come along. If Labour does lose these areas as they have in Scotland they are essentially finished as a viable political force, you can't get even remotely close to winning an election with ideologically left wing voters in big cities alone.
However some constituencies with an industrial heritage have seen their demographics shift dramatically, it is particularly strong in areas where the Conservatives always had a strong base (e.g with rural farming areas or middle class professionals) for example South Derbyshire and North West Leicestershire. As the Labour vote collapsed in the ex-industrial part of these type of constituencies former marginals transformed into safe Conservative seats.
In many Labour coal mining seats new middle class housing private developments are springing up left, right and centre to accommodate the Conservative voting middle classes fleeing the big cities (the pattern in constituency boundary changes of many decades has been the cities and metros losing representation and the 'provinces' gaining). Some ex-industrial towns have also reinvented themselves and have become geared towards more skilled manufacturing or have transformed into commuter towns for large cities as the ex-mineworker/steelworker vote literally dies off, the latter change in particular (given voting patterns in Britain) will wipe out Labour's vote in an area in one foul swoop.
These changes potentially present a huge problem for Labour. While the very deprived ex-industrial constituencies should remain Labour at least in the short term but it would only take the right kind of populist movement to sweep them all away. In many others the demographics are shifting in a very unfavourable way as constituencies turn from an ex-industrial ones into 'Middle England' ones.
I hope that helped.
