Found ACP20b on BADER. First paragraph of the publication goes:
‘On declaration of war or on nuclear exchange, whichever is the earlier, air cadet activities will be suspended. In view, however, of the difficulties in communicating during a War Emergency, it is unlikely to be possible for HQ Air Cadets to issue a formal notification of the suspension of air cadet activities. In the absence of any formal notification, therefore, Regional Commandants, Wing Commanding Officers, Squadron Commanders and Officers Commanding detached flights are to take action to close down their units and to carry out the administrative instructions set out below, as soon as it appears from official announcements in the press or over the national radio and television networks that major war is inevitable.’
Thanks for the responses everyone, especially Baldrick .