That is an interesting question, and one to which I don't have an answer. But it raises the additional question of whether any aircraft involved in paid training is required to have and aerial work CofA and makes me wonder would the insurance therefore be sticky should anything happen. It wouldn't be a problem if say getting checked out in a particular aircraft or when getting some form of refresher/differences training, but when one is clearly working towards achieving a rating....??
As side from the fact that training can only be done at a licensed airfield, you can only do PPL training through a course with an RTF, Registered Training Facility.
Which is not to say it doesn't happen, but the hours would not be loggable towards the licence issue.