Retired doctors may continue to work in health in many different capacities. The need to revalidate depends upon whether you require a licence to practise to undertake any or all of the work that you do. You should speak to the organisation that you are working or volunteering for to ascertain whether that organisation requires you to hold a licence to practise. 

If you do undertake any activity post-retirement that requires a licence to practise (e.g. if you work directly with patients, or if your employer requires it), you will have to revalidate in the same way as all doctors, linking to a designated body and Responsible Officer, participating in annual appraisal and collating a portfolio of supporting information in relation to the work that you do. Please see the GMC’s information on licensing for further information.

You may also find the GMC's 'Revalidation Frequently Asked Questions for Retired Doctors and those preparing to Retire' useful.