Rule 40 of the Rules of the Air:
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.
The manoeuvring area includes the runway.
If an airfield is closed (or open for that matter) it is an offence to land or take off without the permission of the person in charge (or if open, the person in charge or atc/fiso).
I have defended a couple of pilots recently who were charged with this offence. Fortunately they were found not guilty after trials but not before they had been put to not insignificant time and expense.