PICUS/P1.S can only be credited with you as PIC on a test with a CAA examiner, eg CPL skills test, LPC etc.
Sorry, a touch OT: I'm always been slightly curious over this one. The guideline is that when flying with an examiner for the purpose of license reval/renew/issue then you should log the flight as PICUS. This is according to the UK CAA (ref LASORS).
I have been unable to find anything written elsewhere that forbids the use of PICUS under other single pilot circumstances. I've certainly used it for checkouts for which there is no UK CAA recognition and therefore no recognised instruction. Although there is a vague requirement in the ANO for an instructor to log their time as PIC when they act as instructor. The PICUS thing fits these circumstances quite nicely.