The challenge is simple. The French believed that Hitler would simply do the old school thing and swap a few provinces with them and end the war, much like back in 1870.
What if Hitler actually did this? What if he agreed to take Alsace-Lorraine but leave the French Gvt. intact in exchange for peace?
Basically, Hitler does not occupy all of Frances and set up Vichy but rather, like in 1870, takes A-L and ends the war there. Perhaps he holds onto the Netherlands but withdraws from Belgium.
Do you think any of this would be acceptable to the British?
I don't think the doctrine of unconditional surrender factored into the Allies at this point.