It has been long established that the IMC allowes the flight under VFR out of site of the surface in countries where this is permitted. It is actually a UK restriction on a JAR PPL that prevents flight out of site of the surface without an IMC.
It is perfectly legal to fly above cloud in France including in the airways VFR.
Your problem of course is that you have to find a hole in the cloud to get back below as flying an Instrument approach can only be done under IFR and that is illegal without an IR!