My 6p: for negligible/minimal use it is handy to keep your number and avoid the hassle of swapping SIMs. If, on the other hand, there are loads of people who will be calling you (and/or you want to make lots of calls) then it is better to get a local card.
One big gotcha: make sure your phone does NOT divert calls to voicemail on busy or unavailable!! If it does, then if someone calls you and you don't answer you not only pay for a call from UK to France, but you also pay for another call from France back to your voicemail number in Uk! For this reason I very rarely set any diverts on my phone.
Only 3.5 days till departure