Paint is very expensive in France, much cheaper and easier to use from UK (or is it that I’m used to British paint?).
I bought some French ceiling/wall paint a few years back and it was like painting with milk - got more on me and the floor/walls than stuck to the ceiling!
I have found that if you mix French paint with a paddle stirrer attached to a drill it does thicken up (as was liquid at top of pot, sludge at bottom).
That doesn’t address the cost issue though.
Thanks to everyone who responded to my original post. You may like to read the blog post I wrote based on your advice and comments.