And to continue being pedantic from four years ago, I still think people mean north of Watford and not north of the Watford GAP! If you look at a map, the most northerly part of the London metropolis is Watford. Beyond Watford is fields, cows, trees, tractors and scary non cockerneys who talk funny. If the most northerly point of the metropolis happened to be, say, Enfield, the expression would be 'North of Enfield', I believe.
If the north as you and Simsy say, does begin north of the Watford GAP (that place in Northamptonshire) can someone please tell me why this point in particular and say not the Northamptonshire/Leicestershite border for instance. Sorry to be so picky! Lol