I hope the following extract of article 20 of the Air Navigation Order 2000 helps
Composition of crew of aircraft
20 - (1) An aircraft shall not fly unless it carries a flight crew of the number and description required by the law of the country in which it is registered.
(2) An aircraft registered in the United Kingdom shall carry a flight crew adequate in number and description to ensure the safety of the aircraft and of at least the number and description specified in the certificate of airworthiness issued or rendered valid under this Order or, if no certificate of airworthiness is required under this Order to be in force, the certificate of airworthiness, if any, last in force under this Order in respect of that aircraft.
(3)
(a) A flying machine registered in the United Kingdom and flying for the purpose of public transport having a maximum total weight authorised exceeding 5700 kg shall carry not less than two pilots as members of the flight crew thereof.
(b)
(i) Subject to sub-paragraphs (ii) and (iii), an aeroplane registered in the United Kingdom and flying for the purpose of public transport in circumstances where the aircraft commander is required to comply with the Instrument Flight Rules and having a maximum total weight authorised of 5700 kg or less and powered by:
(aa) one or more turbine jets;
(bb) one or more turbine propeller engines and provided with a means of pressurising the personnel compartments;
(cc) two or more turbine propeller engines and certificated to carry more than nine passengers;
(dd) two or more turbine propeller engines and certificated to carry fewer than 10 passengers and not provided with a means of pressurising the personnel compartments, unless it is equipped with an autopilot which has been approved by the CAA for the purposes of this article and which is serviceable on take-off; or
(ee) two or more piston engines, unless it is equipped with an autopilot which has been approved by the CAA for the purposes of this article and which is serviceable on take-off;
shall carry not less than two pilots as members of the flight crew thereof.
(ii) An aeroplane described in sub-paragraphs (i)(dd) or (i)(ee) which is equipped with an approved autopilot shall not be required to carry two pilots notwithstanding that before take-off the approved autopilot is found to be unserviceable, if the aeroplane flies in accordance with arrangements approved by the CAA.
(iii) An aeroplane described in sub-paragraphs (i)(cc), (dd) or (ee) which is flying under and in accordance with the terms of a police air operator's certificate shall not be required to carry two pilots.
The whole Order can be found at:
http://www.hmso.gov.uk/si/si2000/20001562.htm#20