AIC 127/1999 (White 378) is purely advisory. It's the CAA saying "A bit of advice - if I were you I'd do these things". The law, as enshrined in the Air Navigation Order only states that:-
For revalidation of single-pilot single-engine piston aeroplane (land) class ratings and/or touring motor glider class ratings the applicant shall on singleengine piston aeroplanes (land) and/or touring motor gliders satisfy the requirements specified in paragraph 1.245(c)(1) of JAR–FCL 1.
and paragraph 1.245(c)(1) of JAR–FCL 1 states
1) All single-engine piston aeroplane class ratings (land) and all touring motor glider's ratings - Revalidation. For revalidation of single-pilot single-engine piston aeroplane (land) class ratings and/or touring motor glider class ratings the applicant shall on single-engine piston aeroplanes (land) and/or touring motor gliders:
(ii) within 12 months preceding the expiry of the rating :
A) complete 12 hours of [flight time in the class including] 6 hours of pilot-in-command time and 12 take-offs and 12 landings; and
(B) complete a training flight of at least 1 hours duration with a flight instructor. This flight may be replaced by any other proficiency check or skill test for a class or type rating.
There is nothing in the law that states the training flight has to contain anything specific and certainly there is nothing that says it can be 'failed'. Also note that there is nothing in there about it being 'signed off' in the logbook - it just has to be taken.
Now, I'm not arguing what should and shouldn’t be in the flight - I enjoy going out with an instructor from time to time, practising all the stuff that I don't do that regularly and getting feedback from a much more experienced pilot. However, instructors are instructors; they are not examiners and if this were intended to be an exam the people who made the law would have said so.