Yes, you can travel to France provided you are travelling for work.
You need to have a valid Covid (PCR) test certificate that's been done within 72 hours before your arrival into the country.
A letter from your employer is not required but it's nice to have, however you do need to fill out the paperwork here
https://www.diplomatie.gouv.fr/en/co...als-in-france/
Also, don't forget you will also need to fill in the passenger form on line before your return to the UK.
https://www.gov.uk/provide-journey-c...fore-travel-uk
Again, as you're flight crew you're except from quarantine.
I live in Belgium but regularly travel to France and the UK for work, the only problem I've had was the 2 hour queue at the French side of the Channel Tunnel the other week. I believe this was due to many people having not filled in the correct paperwork before traveling to the UK.
I've been stopped a couple of times in France, normally at the péage (toll) booths on the autoroute, again it's never been a problem with the correct paperwork.
I hope this helps