Watford Gap

Proper noun

 * 1) A pass between hills in the English Midlands, near the village of, Northants, crossed by the M1 motorway and West Coast Main Line.
 * 2)  The notional border between the North and South of England; the North–South divide.

Usage notes
The Watford Gap is south of Birmingham and most Midlands cities, which are culturally distinct from Northern England. For this reason, the expression is mostly confined to speakers in Southern England for whom these differences are less apparent.