The real anger compared to other countries stems from the Thatcher era IMO.
As a PP says the industries that Northern towns depended on were purposefully decimated in the 1980s with very little support to retrain or incentives for other types of employment.
The result was catastrophic. I really can't articulate how bad things were. Generations of the same communities all out of work at the same time with very little hope of alternative employment.
At the same time massive investment into services in London (banking, insurance, etc) were happening around London. Lots of power suited 'yuppies' as they were called then making obscene money in the City and spanking it on all sorts of shite.
Obviously this wasn't the average Southerner but the stereotyping in the media stuck.
Now most wouldn't associate their views with this period but IMO it's where the anger came from and has passed down a generation or so particularly due to the ongoing lower investment in the North, the London centric media and politicians, etc.
It doesn't help that anyone who hasn't ventured beyond their own regions clings to stereotypes as though they are actually true. There is a grain of truth in the stereotypes of both sides but equally a lot of utter bollocks.
I say this as someone who grew up in the North until 22 and then has lived in the Home Counties and worked in London since then.