The UKs manufacturing industry was destroyed by Countries that were in a position to offer cheaper alternatives, the simple truth is that we could buy the products cheaper abroad, cheaper steel products, cheaper warships, cheaper aircraft, cheaper coal, cheaper everything, the consumer wants the cheapest price, occasionally we make a fuss about how something is made and if the workers producing the garments or the coffee are paid a fair wage but for the most part we don't think about were it came from or to what cost to the indigineous population of the Country we bought it from.
Unions wanted us to buy the dearer products from the UK and quite rightly so, that was thier job, the government (all of them) wanted to balance the books and get products at the best possible price.
When you offer a bag of coal to someone that cost £20 to produce but can only sell for £10 something has to be looked at, add to that someone abroad offering the same bag for £5 and there is a situation to be looked at.
Unions were making demands that NO government could hope to agree to if they were to give the best service to the Country as a whole. Demands that even many of thier members didn't believe they wanted or needed, Governments did not look at alternatives to simply buying everything from abroad, there was a balance to be looked at, a £5 bag of coal, is on the surface cheaper and a wise move, but not if the bag of coal has hidden costs of another £5 for unemployment benefits etc, profitable mines could have stayed open, non-profitable mines could either be improved or closed, certain ships could have been built in Britain whilst importing a part of our fleet.
I don't know all the answers but it seems that everyone governments and unions alike just drew a line in the sand and would not compromise or negotiate.