Second question first - There is no difference between a UK (not JAA) PPL (or CPL or ATPL) and any other ICAO licence and this is the nub of the problem. Each, in broad terms, is valid only for use in the airspace of or the aircraft of the country of issue - it's not quite as simple as that but I don't have all night!
Prior to the introduction of JAR-FCL and mutual recognition, it was necessary to have a national licence validated by the Authority of another country in order to fly aircraft registered in that or a third country. This is still the case for pre-JAR licences and so you will need to have your UK national PPL validated by the DGAC before you may hire a French registered aeroplane in France