Thank you Whopity. It then appears that the worst case penalty for the JAA PPL expiry is the skills test.
What is the minimal route for preventing having to reset the IMCR test? For a current pilot, it's easy enough to take the IMCR skills test every 2 years and combine that with the PPL skills test. On that basis, can one keep going, or does one have to send off the payment for the JAA PPL every 5 years too, and get a CAA medical in order to keep the JAA PPL valid in order to keep the IMCR from expiring?
Still, having to reset the IMCR skills test after 10 years is not a huge burden, in return for having saved 2x the JAA PPL fee and 5x the CAA medical fee.