This was purely a Dental Council regulation, anyone at the time could call themselves doctor, it wasn't a criminal offence.
With the arrival of dentists from EU countries who were allowed to do so, UK dentists felt they were being put at a disadvantage in the eyes of the general public.
The GDC eventually relented with an announcement to the effect that dentists doing this would not be taken action against any longer.
But they clearly didn't approve!