https://brexit.gouv.fr/sites/brexit/accueil/vous-etes-britannique-en.html
“How to prepare for Brexit?
Following the ratification of the withdrawal agreement, the United Kingdom’s exit from the European Union took effect on 1 February 2020. This agreement provides for a transition period to run until 31 December 2020, during which time all of your rights as a British citizen will be upheld.
Pursuant to the withdrawal agreement, you will only be required to hold a “withdrawal agreement” residence permit as of 1 July 2021.
Due to the health crisis, the opening of the https://contacts-demarches.interieur.gouv.fr website, initially planned on 1st July 2020, has been postponed to the 1st of October, 2020.
Also, in accordance with the withdrawal agreement, British citizens without dual nationality are no longer eligible to vote or stand in local (élections municipales) and European elections held in France as of the withdrawal date”