Take a look at Rule 20, CAP 393 Air Navigation: The Order and the Regulations
Notification of arrival and departure:
20 (1) The commander of an aircraft who has caused notice of its intended arrival at any aerodrome to be given to the air traffic control unit or other authority at that aerodrome shall ensure that the air traffic control unit or other authority at that aerodrome is informed as quickly as possible of any change of intended destination and any estimated delay in arrival of 45 minutes or more.
(2) The commander of an aircraft arriving at or departing from an aerodrome in the United Kingdom shall take all reasonable steps to ensure upon landing or prior to departure, as the case may be, that notice of that event is given to the person in charge of the aerodrome, or to the air traffic control unit or aerodrome flight information service unit
at the aerodrome.
(3) Without prejudice to the provisions of rules 27 and 31, before taking off on any flight from an aerodrome in the United Kingdom, being a flight whose intended destination is more than 40 km from the aerodrome of departure, the commander of an aircraft of which the maximum total weight authorised exceeds 5700 kg shall cause a flight plan containing such particulars of the intended flight as may be necessary for search and rescue purposes to be communicated to the air traffic control unit notified for the purpose of this rule.
(4) Without prejudice to the provisions of rules 20(3), 27 and 31, the commander of an aircraft who intends to fly or who flies across any boundary of airspace notified as either the London or Scottish Flight Information Region other than the boundary common to each, shall before so flying, cause a flight plan, containing such particulars of the intended flight as may be necessary for search and rescue purposes, to be
communicated to the appropriate air traffic control unit within the London or Scottish Flight Information Region.