the CAA's guidance on language proficiency, an examiner can only assess at RT exam or flight test - not revalidation by experience
This is not correct.
Pre-EASA I am unaware of any guidance on how an Examiner could assess a pilot as being English Level 6 - and many were done as the Examiner signed off the Revalidation by experience.
When Part-FCL came in the above wording did appear. But the CAA soon realised this was impractical (particularly with the volume of Licence Conversions) and did, and still do, allow Examiners to assess Language Proficiency via a Face to Face, in person, meeting.
Unfortunately I do not believe this 'guidance' was ever officially promulgated, but simple left to permeate down the 'grapevine'.