I think the original phrase was "North of the Watford Gap", but people often mis-abbreviate to "North of Watford". The Watford Gap is not the same place as Watford. It's about 75 miles north of London. Watford is within London's motorway ring road (The M25), and on the underground map. Its only about 17 miles away from the centre.
On the other hand, Balham was the "gateway to the south" (which reached as far as the Mediterranean).