The holder of a JAA PPL can fly VFR.
Flying VFR requires the pilot to remain in Visual Met Conditions (VMC).
Provided that the country where the pilot is flying considers flight above overcast cloud to be in VMC then it is legal for that pilot to fly in such circumstances.
When flying VFR on top, the weather must be such that the pilot can navigate accurately as well as descend to land;
a) At the destination
b) At any time in an aircraft should an engine failure require descent.
Thus in a single engine aircraft, and JAA PPL, flying above 8/8 Overcast cloud cover would only be legal up to the point where the engine failed and flight in IMC became imminent.
This is different to flying across water in a single engine because ditching is perfectly legal. Flying through cloud without an IR (IMC in UK) is never legal.
I have not mentioned specific countries since the JAA rules are the same in every country provided that a JAA licence (not any form of national licence old or new) is held. However, remember that while licencing rules may be harmonised, VMC criteria are not.
Regards,
DFC