The medical thing
Having a valid medical to revalidate is not stated in the requirements for revalidation nor renewal.
However, the Chief Examiner has said that we should check medical currency when revalidating. That guidance would be in accordance with common sense. What hasn't been stated is what we should do should we discover a medical is expired.
I do not know what authority that I have to refuse a revalidation request when the medical is invalid. Therefore I simply treat the occasion as an opportunity to make sure that the pilot knows that their medical has expired. It is now up to them. They would certainly not be able to hire a club aircraft until they can show a current medical.
The CAA do state that a licence will not be renewed without a current medical. Nothing is formally stated by the CAA that any revalidation or renewal of a rating should be declined.