I don't think people who haven't had negative experiences should ignore the people who have.
I am a northerner (north west), living in the south east. I've seen it go both ways but it does happen.
I've lived (for minimum of a few years in each) in Merseyside, London, Bath, Hertfordshire. Low on Scottish/Welsh/NE/Midlands experience though.
At school, up north, people did take the pee out of the one southerner for his accent.
Down here, I have had colleagues mocking my accent and my vowels and occasionally calling me a thief for ten years. It is boring. But it does happen.
Can't remember any negativity while in Bath.
Ime some northerners are negative about southerners because they are assuming southerners think they're better.
Ime many southerners have never been up north and judge it by films and tv that focus on 'grimness'.
Having lived in a few areas of the UK I think it is pretty much the same all over. Areas of depreciation in most big cities. Nice areas in most places. The average large three bed semi will be on a street that look pretty similar to any other with similar type houses on. All that changes is the accent (massively generalising).
OP started with some vagueness but their experiences should not be dismissed.