From the ANO:-
Simulated instrument flight
23 (1) An aircraft shall not be flown in simulated instrument flight conditions unless the conditions in paragraph (2) are met.
(2) The conditions referred to in paragraph (1) are as follows:
(a) the aircraft is fitted with dual controls which are functioning properly;
(b) an additional pilot (in this rule called a 'safety pilot') is carried in a second control seat of the aircraft for the purpose of providing assistance to the pilot flying the aircraft; and
(c) if the safety pilot's field of vision is not adequate, both forwards and to each side of the aircraft, a third person, who is a competent observer, occupies a position in the aircraft from which his field of vision makes good the deficiencies in that of the safety pilot, and from which he can readily communicate with the safety pilot.
Practice instrument approaches
24 (1) An aircraft shall not carry out an instrument approach practice within the United Kingdom if it is flying in Visual Meteorological Conditions unless the conditions in paragraph (2) are met.
(2) The conditions referred to in paragraph (1) are as follows:
(a) the appropriate air traffic control unit has previously been informed that the flight is to be made for the purpose of instrument approach practice; and
(b) if the flight is not being carried out in simulated instrument flight conditions, a competent observer is carried in such a position in the aircraft that he has an adequate field of vision and can readily communicate with the pilot flying the aircraft.