Following on from being northern, it led me to think about where 'the south' begins. (Berwick if you're Scottish). For the English, it's a bit more tricky.
Real southerners talk about north of Watford. North of the Trent was more credible. I just wonder if it's where you start pronouncing 'bath' and 'grass' with a short 'a'. I used to live in Coventry (predominantly short 'a') and often went to Leamington Spa or Kenilworth (a lot more long 'a') so the boundary must be somewhere near Stoneleigh or Bubbenhall in Warwickshire.
Of course, the border isn't straight, so other candidates for north/south would be interesting. I suspect Cornwall has more in common with the north than with Surrey