Hi there,
A couple of quotes from CAP393:
‘special VFR flight’ means a flight made at any time in a control zone which is ClassA airspace, or in any other control zone in Instrument Meteorological Conditions or at night, in respect of which the appropriate air traffic control unit has given permission for the flight to be made in accordance with special instructions given by that unit instead of in accordance with the Instrument Flight Rules and in the course of which flight the aircraft complies with any instructions given by that unit and remains clear of cloud and in sight of the surface;
Choice of VFR or IFR
22
(1) Subject to paragraph (2) and to the provisions of rule 21 an aircraft shall always be flown in accordance with the Visual Flight Rules or the Instrument Flight Rules.
(2) In the United Kingdom an aircraft flying at night:
(a) outside a control zone shall be flown in accordance with the Instrument Flight Rules; or
(b) in a control zone shall be flown in accordance with the Instrument Flight Rules unless it is flying on a special VFR flight.