Even with civil airfields all is not totally clear cut as shown by the following extracts from the UK-AIP.
Fairoaks - ATZ hours coincident with AFIS or A/G hours.
Chichester - ATZ hours co-incident with AFIS hours but not by arrangement.
Both airfields have published normal opening hours, but with a longer period avialable "by arrangement". At Fairoaks the ATZ exists during the "by arrangemant" hours if AFIS or A/G is available, however this is not the case at Chichester (Goodwood).
Farnborough - ATZ is hereby notified as being active H24 for the purposes of Rule 39 [note - Rule 45 in the current ANO] of the Rules of the Air Regulations 1996.
Pilots wishing permission to transit the Farnborough ATZ should in the first instance attempt to obtain that permission from Farnborough ATC on 125.250 MHz during the notified operating hours of that frequency. Outside these hours, contact Farnborough ATC by telephone prior to take-off.
Warning - Pilots must not enter the Farnborough ATZ during notified hours without prior permission.