The company is definitely wrong and the TM wants his ■■■■ kicked for risking infringements for his drivers with this rule, it needs to be within 6" (150mm) of the actual height of the vehicle
From 1st October 1997, if the overall travelling height of a motor vehicle or trailer exceeds three metres there must be a notice in the cab…’
It must:
be where it can be easily read by the driver and have numbers at least 40mm tall
indicate the overall travelling height (to a tolerance of plus 150mm)
be in feet/inches or in both feet/inches and metres (but not in metres only!).
(If you’re going to do both the heights indicated must not differ by more than 50mm.