Not just any old FI, though - they will need to be individually authorised to do this.
2.10 Revalidation of SEP and TMG ratings by FI(A) and CRI(A)
A new paragraph FCL.945 is to be added to make provision for instructors who conduct the training flight for SEP or TMG revalidation by experience to be individually authorised by the Competent Authority to endorse the revalidation page of the pilot’s licence. This will enable the CAA to introduce a Part-FCL equivalent to the “Revalidation Examiner Certificate”
previously issued under national rules. Only instructors with the privileges to instruct for the SEP and/or TMG rating will be eligible to be authorised.
I'm not quite sure why that Para 2.10 refers to CRI(A); individuals with only that qualification can't instruct for the SEP/TMG Ratings, so surely are ineligible.