You MUST apply online as soon as you can
From 1st October 2021, it will be a legal requirement for British citizens to hold a "Withdrawal Agreement" residence permit ("Carte de Séjour") in order to reside in France.
The only exceptions to this are:
- British Citizens who also hold another European Union nationality (e.g. French or Irish) - note that if your application for French Nationality is still in progress you do need to apply for a Carte de Séjour
- Those who applied for a "No Deal Brexit" Carte de Séjour during the period that application site was open, from October 2019 to January 2020 - although in that case you should already have received your new Carte de Séjour
- You arrived in France after 31st December 2020, in which case you will not qualify for a "Withdrawal Agreement" Carte de Séjour, but will have to go through the third party country visa request process.
The latest date to be able to apply for the residence permit was 30th June 2021, but the French Interior Ministry has updated their website to indicate that the site will remain open for applications until 30th September 2021.
No indication has yet been given that the requirement to have your Withdrawal Agreement residence permit in your possesion has been extended past 1 October 2021.
French Government website giving the explanation of what is required from 1 January 2021:
Link to the French Government website for applying for your "Withdrawal Agreement" residence permit:
In French: https://contacts-demarches.interieur.gouv.fr/brexit/brexit-demande-titre-sejour/
In English: https://contacts-demarches.interieur.gouv.fr/brexit/brexit-residence-permit-application/