From CAP 694 "The UK Flight Planning Guide":-
Booking Out
Rule 17 of the Rules of the Air Regulations 2007 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 an FPL constitutes compliance with this Rule. The action of "Booking Out", however, does not involve flight details being transmitted to any other ATSU.
Chapter 1 Page 4 para 5
Ok this Rule no longer applies but this seems to confirm there was a requirement to book out.