In the same way that Labour were at the helm when the financial crash of 2008 hit...The "decline" of industries that sustained 'the north' was inevitable regardless of who was in power. Indeed, the preceding Labour and Tory governments in the 70s were already closing or running down industries at a significant rate. Manufacturing as a whole didn't decline under Thatcher.
The un-paraphrased / non-mythological version of events...
http://davidaslindsay.blogspot.co.uk/2009/05/thatcher-and-scotland.html?m=1