issi noho's reference is a little out of date. The current UK legislation is Rule 48 of the Rules of the Air Regulations 2007.
Originally Posted by Rules of the Air Regs 2007
Failure of navigation and anti-collision lights
48.—(1) Paragraphs (2), (3) and (4) shall apply to aircraft in the United Kingdom.
(2) An aircraft shall not depart from an aerodrome if there is a failure of any light which these Rules require to be displayed at night and the light cannot be immediately repaired or replaced.
(3) Subject to paragraph (4), if the aircraft is in flight and any such light as is referred to in paragraph (2) fails and cannot be immediately repaired or replaced, the aircraft shall land as soon as it can safely do so, unless authorised by the appropriate air traffic control unit to continue its
flight.
(4) An aircraft may continue to fly during the day in the event of a failure of an anti-collision light provided the light is repaired at the earliest practicable opportunity.
Now I'm not a lawyer but I've worked with some and I find paragraph 2 rather amusing. No doubt some lawyer will one day argue that an aircraft is permanently grounded if it doesn't have a system to immediately fix a broken nav light!