These were built to Home Office specification and painted in drab green, like the fire engines, for the Auxiliary Fire Service.
(The AFS were originally formed in 1938 to supplement local fire brigades during WW2.)
In total, 457 of the type pictured here were built between 1952 to the early 60's.
Their purpose was to escort mobile fire columns around the UK in the event of an armed conflict and they were to be ridden by AFS dispatch riders who would clear the way for the Green Goddess fire engines etc.
When they were declared surplus to requirements some found their way into police service. As a token gesture, the tanks were painted white. Restorers usually put them back to original colours and markings.
Engine is a 348cc single 4-stroke OHV delivering 16BHP at 5600 rpm. Top speed 78mph.