The cheapest way to hire a vehicle in France is to hire one from a supermarket.
In Brittany, Super U and Leclerc have some for rental. They have 5 seaters (Fiat Punto for example), 9 seaters, various size vans from 6 to 20 cubic meter, etc.
They will also let you take it to the UK which is very practical. I have already booked a van for a client to take to England for a few days. The price was very reasonable. It is a cheap way to organise a removal.
Book it early as they might have just one of each size!
I have just got a price for a friend for a 5 seater for 5 days. €121 all in.
Having worked for a car hire company in Jersey, I think it is a good price!