Here's a relevant map men video about this very matter.

https://youtu.be/ENeCYwms-Cc

Roughly north of a line from the ~~Mersey~~ Dee to the Humber. If we use counties then the southern borders of Cheshire, Lancashire and Yorkshire form the line.

It's essentially this:

I highly recommend Rory Stewart's documentary Border Country: The Story of Britain's Lost Middleland if you can find it anywhere as it does a good job of looking at the North and how it is so strongly connected to Scotland, it's really Hadrian's Wall that divided us along an arbitrary geographical feature because it was easy to defend.

edit: as much as I'd like to exclude Cheshire I am allowing them into the North, so changed Mersey to Dee.

I drove past Watford Gap yesterday, that felt like leaving the South.

North of the Thames. This means I can claim I moved from South to North.

In actuality the line is somewhere above Nottingham but below Stoke.

North of the thames

North of Northampton because I don’t believe in the Midlands

Yorkshire and up for me

North of Northamptonshire, as someone in the South

I’m a child from a broken home. One parent lived in Yorkshire, the other in the midlands. When they did handover, it was at Donington Park services, the approximate midpoint. Therefore my North begins not far north of Donington Park services.