Since the French Gov have decided to give the "choice of Birth Nationality (worldwide) re Inheritance"..for its French Residents...it makes no difference what UK does re Brexit... worldwide means just that..
I am a French Resident... I can make a choice re Inheritance... and I have done so...now I intend to live forever
"EU rules however allow you to choose that the law of your country of nationality should apply to your succession – whether this is an EU country or not." .. a quote from this useful site..