In general, it would be preferable for an appraiser to be a physician (the same medical sub-specialty may not be possible). However, this is not mandatory and will largely depend upon the decisions of the employer, and the availability of trained appraisers within the specialty. 

Your appraiser will need to ensure that they have sufficient information about you, your specialty, and specialty standards to complete the appraisal. Specialty information and advice is available to appraisers and appraisees, including through each specialty’s guidance and through the specialty helpdesks.

The RCP has produced physician specialty resource guides for revalidation which are available from their website. These are designed to help both doctors working within the specialty, and appraisers.