Well, the two fingered version developed during one (which I can't remember - there were so many) of the English-French wars a couple of centuries ago. When the French captured an English archer, they would sever the index and middle finger of the archers hand, making it impossible for him to draw a bow. Hence the (uncaptured) British soldiers taunted the French by holding up their two fingers. I'd always assumed the one-fingered version developed from there, somehow.