I am sure there is no legal need to log a flight in your personal logbook. If you forget to log a flight, that is not a crime.
Eh? My recent copy of CAP 393 (Section 1 Part 4 Pgs 12-13) says this:
Personal flying log book
35 (1) Every member of the flight crew of an aircraft registered in the United Kingdom and every person who engages in flying for the purpose of qualifying for the grant or renewal of a licence under this Order shallkeep a personal flying log book in which the following particulars shall be recorded:
(a) the name and address of the holder of the log book;
(b) particulars of the holder’s licence (if any) to act as a member of the flight crew of an aircraft; and
(c) the name and address of his employer (if any).
(2) Particulars of each flight during which the holder of the log book acted either as a member of the flight crew of an aircraft or for the purpose of qualifying for the grant or renewal of a licence under this Order, as the case may be, shall be recorded in the log book at the end of each flight or as soon thereafter as is reasonably practicable, including:
(a) the date, the places at which the holder embarked on and disembarked from the aircraft and the time spent during the course of a flight when he was acting in either capacity;
(b) the type and registration marks of the aircraft;
(c) the capacity in which the holder acted in flight;
(d) particulars of any special conditions under which the flight was conducted, including night flying and instrument flying; and
(e) particulars of any test or examination undertaken whilst in flight.
(3) For the purposes of this article, a helicopter shall be deemed to be in flight from the moment the helicopter first moves under its own power for the purpose of taking off until the rotors are next stopped.
(4) Particulars of any test or examination undertaken whilst in a flight simulator shall be recorded in the log book, including:
(a) the date of the test or examination;
(b) the type of simulator;
(c) the capacity in which the holder acted; and
(d) the nature of the test or examination.
Compliance with the ANO is a legal requirement, so I would think that the regulations about keeping a personal log book are not "optional".
I don't think this has ever changed under JARs, EASAs, LASORs or Owsyerfathers.