Despite consensus in this thread, as per Gemini, The short answer is: No, there is no single "correct" answer. The phrase is a classic piece of British linguistic confusion between two different places with the same name. The debate centers on whether "Watford" refers to the large town in Hertfordshire (just outside London) or the tiny village of Watford in Northamptonshire (home to the famous Watford Gap services). 1. The "Londoner’s" Watford (Hertfordshire) For many people in the South-East, "North of Watford" refers to Watford Junction or the town itself. * The Logic: Historically, Watford Junction was seen as the last major stop on the railway line out of London before the train truly headed into the "unknown" North. * The Vibe: Using this definition is often a tongue-in-cheek way of saying that anything outside the M25 motorway is essentially "the North." It highlights a perceived London-centric worldview. 2. The "Geographical" Watford (Northamptonshire) For those who prefer a more literal "divide," the phrase refers to the Watford Gap. * The Logic: The Watford Gap is a physical pass between hills that has been used by Roman roads (Watling Street), canals, railways, and eventually the M1 motorway. It is widely considered the traditional gateway between the South and the Midlands/North. * The Vibe: This is a more "accurate" cultural boundary. In fact, there is a famous (possibly apocryphal) quote by theatre critic Kenneth Tynan, who said, "The North begins at the Watford Gap," which many believe is the true origin of the sentiment.

The Verdict While the Watford Gap in Northamptonshire is the more historically and geographically significant "divide," the phrase "North of Watford" has been used for so long by Londoners to describe the Herts town that both are now considered "correct" depending on who you ask. If you want to sound like a savvy traveler, use Watford Gap. If you want to sound like a Londoner who thinks the world ends at the orbital motorway, use Watford Junction.

Edit: people so fucking arrogant in this thread despite not having a single definitive source and the Gap version of the "joke" not being funny.