MP, you should have told the CAA that you were not P1 and quoted the PPRuNe posts last year were quite a lot of the self appointed experts on here were adamant that if you were engaged on a check ride as an instructor with a qualified pilot you could not be P1, funny how none of them have picked up on this!
Need to read the thread a bit more careful next time. I picked upon this in post #5 and the OP responded that he was P1 as per club rules (post #8), and had signed the aircraft out as such (post #1).
In general, on a club mandated club check with a pilot who is otherwise legally current (including the 90-day rule), both the "student" and the "instructor" could legally claim P1, but not simultaneously. So you've got to decide before the flight who is (and gets to log) PIC/P1. At least, that's what I remember from the discussion here a while ago.
In this particular case, the decision seems to have been preempted by club rules. Fair enough. Case closed.