The Drivers’ Hours legislation states that a rest period is a period during which Drivers may freely dispose of their time. It is therefore the compulsory or voluntary nature of CPC training which determines whether or not it should be counted as working time.
Driver CPC training should be treated as working time if you are instructed to attend the training as part of your employment (whether you are paid or not).
If, however, you organise your own training and attend that training during your rest period or day off, then it is counted as rest like any other voluntary activity.