Watford Gap is Village and a break in the hills in Northamptonshire, I wouldn't call it "just north of Watford" - it's almost 100km north.
My favourite motorway sign is at Staples Corner, however "M1 - Watford, THE NORTH."
I don't understand what the whole people from London don't go elsewhere rubbish is about though. It seems to be, living in and talking to people from other large cities myself, most inhabitants of most UK cities never move out, and I'd actually say that people from London seem to be more mobile than others. When I first moved to Bristol to hear half of the class at college declare they had only been to London less than 5 times in their entire life came as a bit of a shock to someone who came from the South East and used to visit other towns & cities on a regular basis! I've experienced and heard the same from other cities such as Birmingham, Manchester & Liverpool too.