The French Foreign ministry maintains a list of countries that officially confirmed they are willing to accept seemingly expired ID cards during their extended period of validity.

  Unfortunately, the UK is not on that list even though the Timatic entry found by @blackbird57 suggests it does really accept those cards.

I have no idea whether it makes a difference one way or the other but the Interior ministry also provides an information leaflet to print and carry with you to show at the border, should the border guards require an explanation about the whole situation. They have similar leaflets in dozens of languages including all EU official languages except Irish, Arabic, Albanians, Catalan, Serbian, etc. for all countries that let French citizens enter without a passport.

The French Foreign ministry maintains a list of countries that officially confirmed they are willing to recognise seemingly expired ID cards during their extended period of validity. Unfortunately, the UK is not on that list even though the Timatic entry found by @blackbird57 suggests it really does accept those cards in practice.

I have no idea whether it makes a difference one way or the other but the Interior ministry also provides an information leaflet to print and carry with you to show at the border, should the border guards require an explanation about the whole situation. They have similar leaflets in dozens of languages including all EU official languages except Irish but also Arabic, Albanian, Catalan, Serbian, Turkish, etc. for all countries that let French citizens enter without a passport.

Personally, I got a passport and I do not intend to ever test this.