Paragraph 23A VFR Plans and air traffic control clearance
Unless otherwise specified by the appropriate air traffic control unit before an aircraft flies within Class B, Class C or Class D airspace during the notified hours of watch of the appropriate air traffic control unit, the commander of the aircraft shall cause a flight plan to be communicated to the appropriate air traffic control unit and shall obtain an air traffic control clearance to fly within the said airspace.
The flight plan shall contain such particulars of the flight as may be necessary to enable the air traffic control unit to issue a clearance and for search and rescue purposes.
Whilst flying within the said airspace during the notified hours of watch of the appropriate air traffic control unit the commander of the aircraft shall:
cause a continuous watch to be maintained on the notified radio frequency appropriate to the circumstances; and
comply with any instructions which the appropriate air traffic control unit may give in a particular case.