Once had a Londoner describe the north as "anywhere north of the M25"
I mean, London's a big city but I've never heard any Londoner say this in all my life living there. It's always been "comedians" usually Northern ones as a cheeky insult to Londoners: "Oh look they don't even know geography". Which, to be fair, we probably don't.
Personally I'd say anything North of Sheffield is da Naaarth. Roughly anything above Wales.