The last post on this seems to be dated 2017 so hopefully someone has some later info or suggestions.
What are the long stay parking options in Roscoff Ferry Terminal or nearby?
I plan to be in the UK for three weeks and getting to and from my village to the Ferry terminal is daunting except by relying on friends for drop off and pickup.
From Brittany Ferries website:
"Parking at Roscoff port is free of charge but vehicles are left at the owner’s own risk. You can park for a maximum of 48 hours in the large open car park in front of the passenger terminal. For stays longer than 48 hours, a permit can be acquired from the Brittany Ferries reception.
There is a small overnight parking area for motorhomes and caravans near the vehicle check-in lanes. You can also find free overnight parking at Sainte Barbe, 1km away from the port, and at Perharidy, 2 km away."
Seems like you might need to contact Brittany Ferries to find out how to arrange the permit.
Sainte Barbe is an open public car park, I used it many times to go to the market etc as I lived across the bay for the past 30 years but, it is risky as is any of the open parking spaces up there. To be honest you might do better posting on a Brittany Forum to see if anyone nearer can have your car for a few days on their property or contact one of the mainhotels who may rent you some space. Perharidy is very isolated, just sand dunes and firs, again I would not park there and leave itfor long!