Just received this gem of an email from La Poste.
Translation of the key points:
As things stand, we would like to draw your attention to the fact that from the actual exit of the United Kingdom from the European Union on 30 March 2019 or any later nominated date:
- you may be required to complete customs formalities for your shipments of goods to the United Kingdom in the same way as you currently do for your shipments outside the European Union.
In order to complete your information for these customs formalities, you can consult our advice here .
- the delivery times of your shipments to the United Kingdom would then be extended due to the new customs formalities applicable when crossing the border between France and the United Kingdom.