The FI has to be CAA as the FI has to sign you off and submit the paperwork to the CAA
Actually it is the EXAMINER not the FI that does the paperwork for the CAA. If you find an FI who is also an Examiner you can do both in one go otherwise you will need to do the 1hr flight with an FI and the paperwork for revalidation is then completed by an Examiner.