FWIW the trend in the vulnerable 'red wall' seats has been towards the Conservatives for decades. This is partly as a result of partisan and class dealignment, and partly because the Labour-leaning forces of the organised working class have been in retreat there since the 70s. A former mining area like Bolsover, for example, has moved to vote more like similarly situated semi-rural, increasingly elderly, and low-wage areas since the pits closed.

Brexit also a factor in '19, of course, but those seats are red in history and not any more in demographics.