Basically the medical requirements are associated with the type of licence - JAR Class 1 for a professional flying licence; JAR Class 2 for a private flying licence. The FI or FI(R) Rating may be added to a private flying licence, but the holder may not receive any remuneration or reward for any flying instruction given (except for flying instruction in Micrlights, Powered Parachutes or SLMGs).
So yes, it would seem that you can be a PPL FI with a JAR Class 2 medical but you must not receive any remuneration for such activity.
For confirmation, in view of your medical circumstances, I would urge you to contact CAA Medical Department to avoid disappointment.
Good luck with your aspirations!