Wow! Wot a minefield, but steady those ambitious enough to try logging supernumerary as P2. I don't believe it!
1) Simple logic. The TRE is P1 and so the stude is P2. No question about that. There cannot be two P2s. So NO, you can't log it as P2 as well.
2) Simple logic. If you were not there you cannot log it either.
3) Simple logic. If FO were under any other test (LPC) and you happened to be on jumpseat you couldn't log that either.
4)
OPERATING CAPACITY
Pilot on flight deck but not as P1, P2, SPO or FE:
(i) Acting as ‘required’ Flight Navigator (under the Air Navigation Order);
(ii) Pilot supervising Co-pilot activities;
(iii) No duties assigned (Supernumerary)
You , as safety pilot, are not "supervising" Co-pilot activities. You are acting as a supernumerary observer to the crew as a whole. I believe the above quote is to cover a trainer in the jumpseat supervising an FO in his capacity as a trg capt (perhaps with a cruise capt in LHS??). A supernumerary observer is not legally required as "crew" by the licencing authority so has no defined "legal" role, therefore cannot log it. (see JetMan's post)
5)
You're not allowed to, but chances are, even if you do log them, no one will ever cross-check yours and your colleagues-in-training logbooks and you will never get caught.
...How Professional.
NOT!
Answer.
NO