Brittany is part of France, not that many Bretons like that fact! So yes, insurance is advisable, the minute you step foot on board Brittany Ferries, a French owned company, you will be treated as a foreigner accessing their health service.
I've take several unfortunate guests to Saint Malo Hospital over the years - broken limbs, cut hands, babies unwell etc - and every time they have asked about insurance beyond the EHIC card. It speeds everything up.
I have a carte vitale in my wallet after many years of working in Brittany and being in the ststem, BUT I still take out UK travel insurance because that is the cover needed.
I hope you have a great trip, the weather needs to pick up a bit because rain is forecast, but I'm hoping for sunshine in 10 days time. If the worst comes to the worst, the Urgences at the hospital in Saint Malo are very good. I don't know about undertakers there thankfully.