I understand that flying IFR isn't possible without a valid Instrument Rating, but must the surface always be in sight while flying VFR (but in IMC or above) in France.
You cannot fly VFR in IMC. Assuming that what you meant in the () was (above cloud), the answer is no. Very few states build "in sight of the surface" into their VFR minima. One that does is, AFAIR, South Africa. Some states do impose extra conditions where VFR is conducted out of sight of the surface.
But it is one hell of a gamble to travel that far and hope there is a gap in the cloud to get back down through remaining VMC at all times!!!
I've been on a flight that climbed above cloud in the UK on IMC-rating privileges and continued over a solid layer towards Duesseldorf (where SCT cloud was forecast). Unfortunately, we encountered lowering visibilites as we approached convective activity around Kleine Brogel, and so we turned round and came all the way back to the UK. The fuel was planned for just that (plus a substantial reserve).
It was a gamble in the pecuniary sense, though I would gladly have paid double for the fuel for access to an in-flight loo.
