Spitoon, try the section on licences.
IIRC, the ANO starts with a regulation that an aircraft may not fly, and then lists a series of exemptions, starting with the word "unless" or some such thing.
One lot of clauses specify what a holder of a PPL without additional ratings can do, like fly VFR in VMC but not (IIRC) in IMC. Another exemption is something like "glider pilots are exempt from the above " (i.e. having licences) for glider (unmotorised) flights starting and ending in the UK and not going over boundaries of other countries. Ditto for C of A, which may mean that any limitation on cloud flying, or requirements for equipment, are put on by the manufacturer, and/or by the BGA if it is flown under BGA rules (which again it does not have to be, in the UK - see another thread). I am sure we do not have to have artificial horizons to be legal.
I really haven't time or motivation to do the research myself - I am satisfied that what we do is well known, well understood, and does not result in prosecutions or other problems.