When all else fails ... read the instructions:
Rule 11 of the Rules of the Air Regulations 2015
(para 1 and table 1 define the aerodromes at which the rule applies, I'm assuming we are at such an aerodrome)
(2) An aircraft must not fly, take off or land within the aerodrome traffic zone of an aerodrome unless the commander of the aircraft has complied with paragraphs (3), (4) or (5), as appropriate.
(3) If the aerodrome has an air traffic control unit the commander must obtain the permission of that unit to enable the flight to be conducted safely within the aerodrome traffic zone.
(4) If the aerodrome provides a flight information service the commander must obtain information from the flight information centre to enable the flight to be conducted safely within the aerodrome traffic zone.
(5) If there is no flight information centre at the aerodrome the commander must obtain information from the air/ground communication service to enable the flight to be conducted safely within the aerodrome traffic zone.
So before taking-off, landing or flying within an ATZ (2), to enable the flight to be conducted safely within the ATZ, we have to obtain permission (ATC (3)) or information (AFISO (4) or AGCS (5)). This has to be obtained before flying within the ATZ ('cos (2) says so) which I guess has to be done by prior arrangement if you're NORDO or, for the rest of us, on the radio. Simples.
Once you're flying in the ATZ:
(6) The commander of an aircraft flying within the aerodrome traffic zone of an aerodrome must—
(a)cause a continuous watch to be maintained on the appropriate radio frequency notified for communications at the aerodrome; or
(b)if this is not possible, cause a watch to be kept for such instructions as may be issued by visual means; and
(c)if the aircraft is fitted with means of communication by radio with the ground, communicate the aircraft’s position and height to the air traffic control unit, the flight information centre or the air/ground communications service unit at the aerodrome (as the case may be) on entering the aerodrome traffic zone and immediately prior to leaving it.
Subsection (c) is problematic if taken literally (what happens if the radio is busy as I'm about to leave the ATZ? Orbit inside until I can tell the controller/FISO/AG operator my position and height?) but in practise I think it's not an issue (glad to be informed otherwise). And it's a good idea to do so because it increases the SA of those other pilots arriving/leaving.
Like @Irv Lee, I too am waiting for the first prosecution of a passenger airliner captain not reporting their position and height as they enter the ATZ at Heathrow. It is a criminal offense not to do so, dontchaknow