Slightly boring answer perhaps, but Danny Dorling, a professor at Oxford, wrote a paper on the subject during his time at Sheffield and came up with his own border between North and South.

It basically starts at the Severn estuary just north of Gloucester (which I expect we'd all consider south) and makes its wiggly way north-east, ending just south of Grimsby (clearly a northern town).

Anywhere above the M4, obviously.

Jokes aside, though, I guess mostly Scotland?

Anywhere north of Birmingham