Pavel,
That is a very simple one - stands for Aerodrome Control Pilot. This was a rostered "duty", very much like "Duty NCO" or "Duty Officer", or "Duty Pilot", and anybody of the right rank and experience could be rostered onto this duty at any time. Generally an ACP would be a GD officer, or an NCO pilot. You can find multiple references to ACP's in various RAF publications, the one I have is the famous little book known as "The RAF Pocket Book" or AP1081, mine is the 1937 edition. The top person in the hierarchy of overseeing night flying at RAF aerodromes was known as "the Officer in charge of Night Flying" (see below). The ACP, despite the general designation, was actually only concerned with the supervision of night flying out on the aerodrome. He was not concerned with the pilots and their briefing, unless they happened to get lost or have an accident on the aerodrome.

To quote AP 1081, Chapter VI, Section 28, para. 319, (P 85):

(i) "Night flying will be carried out under the supervision of an experienced officer known as the 'Officer in charge of night flying'".

(ii) "The whole of the lighting arrangements and the control of the outgoing and incoming air traffic will be under the control of one officer, or NCO pilot experienced in night flying, known as the 'aerodrome control pilot', who will be stationed in the vicinity of the first flare or floodlight."

(iii) "the position of the officer in charge of night flying must be known to the aerodrome control pilot, and means of communication, whether by signal or by orderly (that is, a runner) must be available between them."

Of course these notes were those current in 1937, but much of the equipment was used right through the war. However at large RAF stations in UK which accommodated large bombers of Bomber Command, or Coastal Command aircraft, electric flare-paths might have been introduced at a later stage. However pre-war equipment would remain in use at many of the smaller stations, including training stations throughout the war. This equipment comprised the notorious "goose neck flare", which was like a garden watering cans with a wick sticking out of the spout, and filled with kerosene, and several would be laid out in a row as a reference point and indication of wind direction, and hence take off and landing directions, and a floodlight, often known by its manufacturer's name as the "Chance Light".

David D