Perhaps it would help if the CAA made it clear that in "signing off" a revalidation in the log book, the instructor was not in anyway testifying as to the ability of the pilot, but was simply making a statement to confirm that the actual flight took place. It may well be that instructors are nervous about accepting responsibility for signing off a pilot whom they consider for whatever reason to be unsafe, even though that is thoretically at least irrelevant to the actual purpose of the flight which is simply to impart training.
Bose's philosophy on this whole subject is exactly right, and refreshing to hear from someone who might otherwise have been assumed to be part of the instructor establishment.