However the British Embassy in France is currently advising people to apply for a permanent carte de séjour. These things are less onerous in terms of administration and so less costly. A permanent carte de séjour currently lasts 10 years, which takes us beyond our retirement date. Once we retire we will need to consider whether to stay in France or to return to the UK.
So I've had a happy morning photocopying our passports (every page) and finding proof of address for the past five years. It's quite easily done, in fact, with our council tax bills and the receipts for our rent.