Your passport should be valid for the proposed duration of your stay; you don’t need any additional period of validity on your passport beyond this.
The French authorities have confirmed they will accept British passports extended by 12 months by British Embassies and Consulates under additional measures put in place in mid-2014.
If your passport describes you as a British Citizen you won’t need a visa to enter France. If you have another type of British nationality, you should check the current entry requirements on the website of the French Foreign Ministry and if necessary confirm with the French Embassy.
For stays of longer than 3 months, contact the French Embassy for information about entry requirements.
UK Emergency Travel Documents
UK ETDs are accepted for entry, airside transit and exit from France.