With lots of EU stuff on debate right now this is quite interesting.
I believe two recent incidences occurred, once in WW2 after the fall of France which was suggested by Churchill and stopped after the French surrendered. Secondly in the 1950s with the French president privately suggesting this to the PM.
Was there any real basis to this?
Well, there was of course the Hundred Years' War (1337-1453), in which Edward III of England claimed the French throne on the basis of having been the son of the former king's sister, Isabella, and thus more directly in line of succession than the actual successor Philip VI.
Although technically Edward wouldn't have qualified for the French throne, since the French Salic law specifically forbade succession through the female line (In terram Salicam mulieres ne succedant).
It's a simplification of the reasons for the HYW, but that was the only historical occurence I could think of where it was realistically possible that an English king could have inherited the French throne.
King Henry II did control about half of France.
http://upload.wikimedia.org/wikipedia/commons/3/3d/Henry_II,_Plantagenet_Empire.png
