Lots of good advice above, so I’ll just add the following that hasn’t been mentioned:;tthese are thing s that can take time, but most can be initiated before you leave the UK
i) Get certified copies (apostilles) of important documents such as birth and /or marriage certificates ASAP, Lots of websites say these need to be less than three months old, but at least within our experience, this seems not to have been. the case (even though, I’m adopted, my wife was born in South Africa without UK ancestry and we were married in Jo’burg). These documents will be necessary for many official procedures.
ii) If you’re already, or about to be in receipt of a UK State Pension, let hem know when you move to France so that hey can issue an S! document, which will almost immediately transfer you into the French Health system and enable them to issue the all-important Carte Vitale.
iii) Check when your UK driving licence is due to expire, as the current French application for un pernmit de conduire apparently now has a nine month turn-around, I applied online in January and am still waiting for an acknowledgement that my application has been received.
iv) Set up a French bank account before you move - it seems much easier to do this from the UK. Even if the nearest branch is not very local, once you’ve got a French account, you can more easily switch to a bank that more closely matches your needs.