Is booking out a legal requirement?
It certainly used to be, this from an old "Guide to Visual Flight Rules (VFR) in the UK"
Rule 17 of the Rules of the Air Regulations 1996 requires a pilot intending to make a flight to inform the Air Traffic Service Unit (ATSU) at the aerodrome of departure, an action known as ‘Booking Out’. Filing a FPL constitutes compliance with this Rule. The action of ‘Booking Out’, however, does not involve flight details being transmitted to any other ATSU.
Whether it's been included in the Standardised European Rules Of The Air (SERA) Regulation I am not quite sure but I guess you'd find it there if it is.
This from another:-
• Transition to the SERA rules covering VFR night flying
• Some modifications to flight planning requirements. For example, the UK expects to keep the existing process of ‘booking out’ at an airfield before a flight but will consider whether to introduce some form of ‘booking in’ at the end of a flight
• A change from the use of quadrantal cruising levels to semicircular cruising levels to align the UK with the rest of the world.
Legislation aside I think it is common sense to "Book Out".