Prior to 2015 participants had to prove insurance. In 2015 insurance was arranged for everyone. However, French participants had to be members of the relevant bodies, although the insurance augmented their existing cover. So don't worry.
I have severe reservations about this ..
when organising LEL 2013 I could not get any answer from any of the PBP organisers about who provided their 2011 insurance.
When organising LEL 2017 .. UK insurers would NOT provide any cover for any overseas resident, and I would be concerned that the same might well apply to a French insurance contract .
However if any AUK /UK resident has had a successful insurance claim for PBP 2015 .. I would be delighted to learn that the French did actually mean what they say.
My personal choice would be to organise standard travel cover for 7 days making sure that 3rd party cover was included