For me it’s where people say ‘last’, rather than ‘larst’. Coming from Nottingham I’m in the former camp. But raising our kids in Sussex has brought this subject up a lot. Grass, bath etc are taught as ‘tricky words’ (non-phonic) in southern primary schools…. proving the North is correct 😁
Where is up north for you? When I lived in London it was Walthamstow, now I live in South Norfolk, it's Swaffham or even Hellesdon! I'm only 3 miles from Norwich but going in to the city is heading up north for me