You're thinking of Watford, on the northern edge of London, just inside the M25.
Watford Gap is an entirely separate place in the Midlands, near Northampton, about 60 miles north of Watford. It was traditionally a key coaching stop on the major routes linking north and south (even today it's on the M1, close to the start of the M6). Not even the most optimistic airport owner would try to claim it is part of London, or even that it marks the boundary of London.
Watford Gap's historical status as marking the north-south divide is largely based on the boundaries of regional accents.