There is no legal requirement for a UK citizen to submit an officially-translated British birth certificate with an apostillé for a Carte Vitale application.
BUT, this doesn't mean you won't get hassled.
I did everything correctly and promptly when I applied for my Carte Vitale a couple of years ago but it was only by a stroke of luck that I finally avoided being totally screwed over.
I submitted all demanded documents, sometimes more than once. I received my social security number about 3 months later. I corrected the date-of-birth error on the final form that asks for your photos to get the actual CV and sent everything into CPAM.
At about the same time I received a letter from a different office at my regional CPAM demanding an original English birth certificate with a raised seal, which doesn't exist. I immediately wrote back to this fonctionaire with an explanation and, to be on the safe side, included yet another officially-translated birth certificate.
A couple of months later, with still no Carte Vitale, I decided to test whether the social security and CPAM numbers I had been issued actually worked. They were rejected as invalid numbers for payment of some lab work. The clinic called CPAM and was told my file had been cancelled because I had neglected to submit a required document within the required time frame.
I immediately went in person to CPAM and took along a government-listed official translator to avoid the possibility of annoying anyone with my imperfect French. We were informed that my name could not be found in the system and that no file existed for me.
We politely asked for referral to someone higher up the food chain and a call was made. But everyone was apparently in a meeting so my mobile number was taken and I was promised a callback within a day or two.
I then phoned CPAM in Nimes and the lady who answered told me I had phoned the wrong number. The number she then provided was the same number I had dialled and it was only when I explained that I had gone to the expense of hiring an official translator to accompany me to the local CPAM that she softened and agreed to check into my records. She actually got back to me! The next day there was a message from her saying it had been sorted.
I soon received yet another form to send in photos for my Carte Vitale, this one with the correct date of birth but a new social insurance number, which I immediately mailed in.
A few other notifications arrived with this new social insurance and CPAM number but I wasn't asked to do anything.
My Carte Vitale then arrived by La Poste, with my photo and correct data. All within a month of that phone call to CPAM in Nimes.
No mention was again made of a need for the raised seal birth certificate that does not exist and which had resulted in my entire CPAM application being cancelled without notification and later wiped from the system.
I live in the Alpes-Maritimes and am now terrified of moving and changing departements in case some fonctionaire again deletes me from existence. Call me cynical but I'm wondering if the odds of that happening have now increased with Brexit.
Sorry if I'm scaring you. Send them an explanatory letter. It didn't do much for me but who knows?