Bignell's Corner is the name given to the area immediately surrounding South Mimms services, although some road signs suggest that the entire M25/A1 junction takes this name, rather than the better known 'South Mimms Interchange'.
The name (and the services) date back to the pre-motorway era, where the A1 and A6 met at a busy crossroads, thought to be where this roundabout is now. Scattered around the junction were two pubs, a garage, a garden centre named Bignell's (hence the junction's name) and a large truckstop known as the Beacon Café, which was popular with lorry drivers heading in and out of London and also infamous for its use by prostitutes. 1960s these had all closed, except for the truckstop.
In the early 1970s the modern interchange was built, but it was originally the junction of the realigned A1 and A6/A1178, plus two local roads, one of which looped past the truckstop and on to the village of South Mimms. It was around now that the Beacon Café was taken over by BP and given the unofficial title of 'Beacon Services', presumably because they knew what was happening next.
By early '80s part of the A1 had been upgraded to motorway and the A6 and A1178 had been upgraded to M25. As the M25 came close to completion, the north-east quadrant of this junction (where a motorway maintenance depot had already been set up) was designated and later approved as the site of a potential MSA.
BP moved in as the owner of the services and it became the first MSA on the M25, opening in 1986. BP chose to lease out the main facilities to Trusthouse Forte, opting just to run the petrol station and truckstop. Trusthouse Forte chose to brand the services as Welcome Break, a chain they had just acquired.
The truckstop has since become further integrated into the Welcome Break service area (Trusthouse Forte are no longer involved, and it's not clear whether BP are either), becoming the main lorry parking area on the site, but still boasting features such as secured fencing which other MSA lorry parks don't have. It was recently marked as a Highways Agency approved truckstop.
SABRE Forums: South Mimms Interchange - more information on the junction
Destination Nowhere: A South Mimms Motorway Service Station Diary by Roger Green - more information on the history of the services