Must Britons in France keep a UK passport if not planning to move back?

There are no laws which require Britons abroad to hold a valid UK passport, however it can be useful to have one, for example if you needed to travel urgently to see an ill relative. 

You can obtain an emergency passport in these circumstances however this can be more difficult to obtain for those who have let their passport expire and willingly choose not to renew it. 

Read more: Can I travel to the UK from France without a passport in an emergency?

If you are planning to travel within the Schengen Area, as a non-EU citizen you should take your passport with you, even if you are not entering or exiting the bloc. 

The EU states a country in the Schengen Area can refuse entry to a visitor if they cannot provide a valid travel document, such as a passport, upon request.

You state that you are sure that you will not leave France anymore, presumably not including travel within the EU. Even if this is the case, having a passport can still be beneficial for a

number of other reasons. 

A passport, even a non-French one, is widely accepted as a valid form of ID by the French state services, in cases where other forms of ID may not be. 

In most scenarios within France your carte de séjour will also be accepted. However a passport may be beneficial should you later need to make a request to the UK government for a service.

However, and most importantly, you will be asked for passport details when applying to renew your Brexit WA card (your current French residency permit). 

Read more: Permanent residency cards in France: 2024 renewal rules