You can’t leave the RN alone, as it will sail out of reach of your airforce and subs alone won’t cut it. But you can’t cripple them either without leaving France live for another turn.
Maybe attack all out at sea and leave Paris, but do conquer Normany, Southern France and Yugoslavia (and of course Finland and Bulgaria). France will be able to build for another turn, but you will be able to throw everything at it (except for some Eastern European forces).
I think I will try the latter, because sinking an extra UK BB is worth more in IPC then the complete French income (19). Sinking all trannies is another very expensive blow for the UK and helps you a lot in the long run.