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

https://youtu.be/ENeCYwms-Cc

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

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.

Yorkshire and up for me

North of Northamptonshire, as someone in the South