This is no longer the case, The ANO has been amended, supervised solo is only legal when for the issue of a licence or rating, once you've applied for a licence, it's dual only or stay on the ground until you have received and signed your PPL.
The ANO has not been amended this would have required a Regulatory Impact Assesment, and it has nothing to do with the JAA!
52.—(1) A person may 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 within the United Kingdom, the Channel Islands and the Isle of Man, without being
the holder of an appropriate licence granted or rendered valid under this Order, if the conditions in paragraph (2) are satisfied.
(2) The conditions referred to in paragraph (1) are that—
(a) the person is at least 16 years of age;
(b) the person is the holder of a valid medical certificate to the effect that the person is fit to act as pilot in command, issued by a person approved by the CAA;
(c) the person complies with any conditions subject to which that medical certificate was issued;
(d) no other person is carried in the aircraft;
(f) the person acts in accordance with instructions given by another person holding a pilot’s licence granted under this Order or a JAA licence, in each case being a licence which includes a flight instructor rating, a flying instructor’s rating or an assistant flying instructor’s rating entitling that other person to give instruction in flying the type of aircraft being flown.
Any FI has the discretion to send a person solo if he is over 16 and has a valid medical certificate. The person can only act as a student pilot and could be regarded as receiving continuation training for the SEP. The law is there to stop people from flying solo without a FI being involved not to be bloody minded to people awaiting a licence.