ThomasTheTankEngine See extract from article 21 of the ANO below.
Members of flight crew – requirement for licence
21 (1) Subject to the provisions of this article, a person shall not act as a member of the flight crew of an aircraft registered in the United Kingdom unless he is the holder of an appropriate licence granted or rendered valid under this Order
(b) subject to article 26(3) of this Order, act as pilot in command of an aircraft for the purpose of becoming qualified for the grant or renewal of a pilot’s licence or the inclusion or variation of any rating in a pilot’s licence if:
(i) he is at least 16 years of age;
(ii) he is the holder of a valid medical certificate to the effect that he is fit so to act issued by a person approved by the CAA;
(iii) he complies with any conditions subject to which that medical certificate was issued;
(iv) no other person is carried in the aircraft;
(v) the aircraft is not flying for the purpose of public transport or aerial work other than aerial work which consists of the giving of instruction in flying or the conducting of flying tests; and
(vi) he so acts in accordance with instructions given by a person holding a pilot’s licence granted under this Order or a JAA licence, being a licencewhich includes a flight instructor rating, a class rating instructor rating, a flying instructor’s rating or an assistant flying instructor’s rating entitling him to give instruction in flying the type of aircraft being flown
However a colleague of mine recently told me that there is a recent UK training.com communication from the CAA which relaxes this requirement, but I can't find it yet.