Now that the UK leaves the EU, it has become a 'third country' - a country which is not a member of the EU. This means that from the end of the transition period, and unless derogations are negotiated as part of the future relationship/trade agreement:
freedom of movement will end for British citizens;
you will no longer have the right to live in any EU country unless you go through the full immigration process as a third country national;
unless you have already become legally resident in France by the end of the transition period you will not be covered by the Withdrawal Agreement;
you will be treated exactly the same as nationals of the United States, Australia, Syria, Turkey, Bangladesh and most other non-EU countries, and you will become an étranger rather than a citizen of the EU;
if you want to stay more than 90 days in France you will have to apply for a long stay visa before you travel;
to become legally resident you will have to meet conditions that are in most cases stricter than those for EU citizens, and you may well be subject to language and other integration requirements. Some may not be able to meet these conditions.
Will not be able to benefit via from S1 for heatlhcare cover for retirees from EU to EU.