Up yours, Boyo
As Spleen said, I don't think anyone has any real evidence for the origin of the V sign being Agincourt (or any other longbow-based evidence). It's just another good old story that everyone is convinced is true due to it sounding fairly reasonable and then being trotted out on regular occasions - not unlike the myth about being able to see the Great Wall of China from the moon for instance.
I read somewhere that many or even most of the archers on the English side at Agincourt were probably Welsh not English due to the way army 'recruitment' used to work in those days.
Mine's the flameproof coat.