The expression's original form was "North of Watford Junction", which is on the outskirts of London about a 100km south of the Gap. In the '50s "North of St Albans" was used in London's east-end, not sure I knew Watford Gap even existed before the '70s. It probably shifted to Watford Gap because Watford Junction and St Albans excluded too much Tory Territory… Cotswolds, Chipping Norton etc etc.
It's now thought the Vikings never got beyond Watford Gap before meeting their Waterloo at Stamford Bridge (no, not that Stamford Bridge)
FTR : "North of . . . " is UK rough equivalent to US Mason-Dixon line.
BR