I have never done a revalidation flight where I haven't had to make a suggestion or demo something. The flight is a chance to brush up on skills and if used as such, then the FI is doing some training.
The arguement about P1/S or P/UT only becomes relevant if the FI does nothing, but as no instructor worth their salt would do that, then the argument doesn't really apply.
"Any licence holder flying a single pilot aircraft, or a student undertaking
a course of flying to obtain a PPL can only log PIC/US after a successful
flight test. All other entries should be P1 or PUT. The only other times
PIC/US can be entered is when a commercial pilot is flying a multi pilot
aircraft. The section of LASORS that you refered to in your email relates to
this multi pilot scenario."
An e-mail I got from the CAA about this issue.