For stays of up to 3 months 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. For stays of longer than 3 months, contact the French Embassy for further information about entry requirements.
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.
UK Emergency Travel Documents (ETDs)
UK ETDs are accepted for entry, airside transit and exit from France