The following is copied from the CAA Lasors document, which is related to ICAO Annex 1: Personnel Licencing
Private Pilot’s Licence (Aeroplanes)
Minimum age - 17 years[/left]
No maximum period of validity
Privileges:
1) Subject to paragraph (2), the holder of a Private
Pilot’s Licence (Aeroplanes) shall be entitled to
fly as pilot in command or co-pilot of an aeroplane
of any of the types or classes specified or
otherwise falling within an aircraft rating included
in the licence.
2) He shall not -
...
b) receive any remuneration for his services as a
pilot on a flight save that if his licence includes
a flying instructor's rating, a flight instructor
rating or an assistant flying instructor's rating
by virtue of which he is entitled to give
instruction in flying microlight aircraft or
self-launching motor gliders he may receive
remuneration for the giving of such instruction
or the conducting of such flying tests as are
specified in sub-paragraph ((a) (i)) in a
microlight aircraft or a self-launching motor
glider.
In other words: you can carry passengers, but you cannot earn money with it.
This is the only limitation I could find. I thought to have read somewhere that a ppl holder cannot fly a multi-engine aeroplane, but I must be wrong
Hope this helps
C-T