My understanding is that you cannot log p1/s except after a successful test with an examiner (or when acting as a supervised co-pilot, but this only applies to aircraft which have a minimum crew of 2 pilots, unlike most GA types which require only 1 pilot).
See Irv Lee's FAQ, especially items 24 and 8.
http://www.higherplane.flyer.co.uk/faq.htm
See also the LASORS at
http://www.caa.co.uk/docs/175/Lasors_Section_A.pdf