Yes it will be Ok to use. If you have to delay your return to UK by lets say a few days, you will still be OK to travel on that passport. The simple rule is that ' you dont stop being British because your passport has expired'. Atleast not when you are coming back to the UK. the above answer is valid only for British passport holders.
yeah you'll be fine becuase you're within the EU. Outside of the EU you need 6 months clear on your passport, but you'll be fine to france. Exact same thing happened to me when I went to prague and flight was delayed so passport became expired but there were no probs!