It is an offence to land at or take off from an aerodrome without the permission of the person in charge.
The Rules of the Air provide:
40 An aircraft shall not taxi on the apron or the manoeuvring area of an aerodrome without the permission of either:
(a) the person in charge of the aerodrome; or
(b) the air traffic control unit or aerodrome flight information service unit notified as being on watch at the aerodrome.
Article 255 of the ANO defines Aerodrome as:
'Aerodrome':
(a) means any area of land or water designed, equipped, set apart or commonly used for affording facilities for the landing and departure of aircraft; and
(b) includes any area or space, whether on the ground, on the roof of a building or elsewhere, which is designed, equipped or set apart for affording facilities for the landing and departure of aircraft capable of descending or climbing vertically; but
(c) does not include any area the use of which for affording facilities for the landing and departure of aircraft has been abandoned and has not been resumed;
The definition is very wide and would appear to include a private strip if it is land "set apart ... for the landing and departure of aircraft". A farmer's field or a disused airfield would not appear to be within the definition but then there are issues of trespass.