It is in fact perfectly reasonable to depart in the UK IFR, climb above the cloud, and so long as one is VMC on top when crossing the FIR boundary to France, that's OK, and typically 1/3 of the way down through France the cloud vanishes.
Similarly on the way back.
Of course these conditions aren't always like that, but often they are, and when they are it is easy enough to check that they are likely to remain so. One can get TAFs and METARs and god knows what else for all of Europe via e.g. Avbrief.