I think it was endorsed by Churchill, and would likewise have been by Reynaud, with the tacit understanding that the Union was only for the duration of the war. Although, as Wendell says, it certainly would have made for interesting debates after Germany's surrender. After all, Jean Monnet was a lifelong supporter of supranational European integration, and in the post-war years devoted his considerable skills to setting up the European Coal and Steel Community, which by 1957 had become the European Economic Community (and by 1993 the European Union). He certainly would have lobbied both governments into making the French-British Union the core of a Europe-wide protofederal entity; whether he would have been successful, though, is anyone's guess.