The regiustration requirements are mainly around putting numbers on the drone and a basic airspace law/safety test. The former is easy and the latter is pretty well all common sense. The model flyers have "acheivement schemes" (once called "A license and B license") which show basic levels of piloting competence plus a knowledge of the relevant regulations. The B-license adds what you might call an "aerobatic rating" to show ability to handle aeroplanes in manoeuvres and unusual attitudes - this was originally the qualification for competition flying and flying at displays. The achievement schemes were originally introduced to satisfy the insurers about basic levels of competence, and due to the regulation/safety questions have now been acceptapted by the CAA as well exceeding the requirements of their own tests, so association members who hold these certificates are exempt from the requirement to pass the test. There is a small fee to cover the costs of the CAA database of registered flyers.
I would expect a similar exemption would be available to PPL and higher for the same reason - the understanding of regulation and safety is well in excess of the need. But someone needs to actually sit down with the CAA and thrash out the details. Unless/until they do it's just a matter of passing the on-line test and paying the fee (£16 IIRC). The ytest is very, very simple. Any PPL-holder who fails the test should be wondering why!
PDR