Revalidation and Renewal of JAR Medical Certificates | Medical | Safety Regulation might help you.
You will get an eye test at renewal but it's very basic for Class 2. All the opthalmological stuff is done on initial only. That said, the CAA recommend you take your current prescription to the AME if you wear glasses or contact lenses - as it says here:
Initial JAR Class 2 (Private Pilot) Medical Examination | Medical | Safety Regulation That's for the inital examination but I would take it for a revalidation too, just in case.
Just for information - Class 1 is different. Your eyes are tested at the medical too but you also have to supply an optician's report every 2 years at least.