When flying under JAA-rules: Never! Your logbook is an official document that has to be kept according to set rules.
But not in the UK. JAA "rules" have no legal status except those that have been implemented via the Air Navigation Order.
A log book is required in accordance with Article 35; it states what a pilot must log however, it is a Personal Log book, and so long as the requirements of Article 35 are met a pilot can put in it whatever additional information he/she likes.
As a passenger you are not subject to article 35 and what you write is up to the individual. The important thing is that it is not able to be confused with pilot time.
The document listed above is for additional ratings, not for the PPL, details of which can be found here:
http://www.caa.co.uk/docs/33/SRG_FCL_19_A.PDF