If you look on some of the other threads about driving licences you will see that Nantes has suspended licence exchange for the time being. So if none of the exceptions apply to you, you might as well hang on and save your self a stamp as most possible the application will be sent back.
See here
I had added this to the running posts about exchanging licences but thought maybe it needed a separate post. This is taken from a Facebook page about exchanging licences and you can check the info on the Loire Atlantique website
.DRIVING LICENCE APPLICATIONS UPDATE - NANTES
All applications are to be put on hold, unless your licence is running out or a renewal is required until the terms of Brexit have been decided. This is the translated text and I will put the link to the page at the bottom.
UK license exchange in French license? Patience pending the decision on the exact terms of Brexit
Updated on 28/02/2019
British license exchange in French license
Do you have a British driver’s license that you wish to exchange for a French license? It will take a little patience and wait for the decision on the precise terms of Brexit to request your license exchange. Only applications that are subject to license renewal due to loss or theft, or mandatory exchange clauses are currently be…