Why would you want to convert a lifelong licence into one that you have to pay to renew every five years?
Correct me if I am wrong, but as I understand it, if you apply for a JAA-PPL that does not mean that you lose your old CAA-PPL. You just hold both licences.
But having said that, as your old CAA-PPL is a recognised ICAO licence, it should enable you to hire in most countries.
But, don't let me stop you donating more money to the CAA!
D_B - still determindly clinging on to her old CAA-PPL