DB I would guess (only a guess cos I'm no lawyer) that with a UK PPL and IMC you could fly VFR "on top" in France as well as the UK. My logic is that the rule requiring you to be in sight of the surface is a basic PPL licence restriction, not a restriction in the definition of VMC. Once you have an IMC rating, this restriction is removed from your licence.
VMC does not require you to be in sight of the surface unless you are taking advantage of the allowance for the visibility to be down as low as 1500m PROVIDING you are at less than 3000', less than 140 knots, and IN SIGHT OF THE SURFACE. Under these circumstances visibilities as low as 1500m are still VMC. I think this mention of "in sight of the surface" is what confuses a lot of people into thinking all VMC has to be in sight of the surface. In fact it is only in this special case.
For simplicity I have not listed all the VMC criteria for different altitudes and classes of airspace.