I have a service bulletin T-65-15 about grounding. It is 3 pages long and starts with:
General
Current practice on vehicles of British design is to ground the positive
terminal of the battery, while other countries have retained, or reverted to,
the use of negative polarity for the ground circuit.
With the increasing use of polarity sensitive devices, such as silicon diodes,
transistors, and electronic components in vehicle electrical equipment, there
are advantages to be gained from the early adoption of a standardized vehicle
grounding system, particularly in the service field.
The reasons which originally prompted the adoption of positive ground on
British vehicles, although still being valid, are now of less practical
Importance owing to improvements in electrical design. In the interest of
standardization most vehicles manufactured in England will in the future change
over to a negative ground system.
Two manufacturers have already introduced production models with negative ground
equipment. These are the David Brown 3 cylinder, 880 implematic tractor, with
conventional D.C. equipment and the Jaguar 4.2 litre Mk. X and E-types with
A.C. equipment. This trend will undoubtedly continue until eventually all
British vehicles have negative ground electrics.