Yes, it's amazing what one finds out when reading the ANO Art 20 sect 1/23
"(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 pressuring 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 notwhistanding that before take-off the approved autopilot is found to be unserviceable, if the aeroplane flies in accordance with arrangements approved by the CAA."
So, in other words, an autopilot is required. Should it be found unserviceable, then some arrangements may apply...