Why do the British swear with two fingers instead of one middle?

It derives from the Battle of Agincourt where the French claimed they would cut off the arrow-shooting fingers of the English archers and bowmen.The English won the battle and flicked the two fingers at the French as a salute in victory.Since then it's been used as an insulting and derogatory gesture.

Actually we are quite good with both. The single finger always seems somehow more rude.

Because they're British. You can't say anything else. They're just BRITISH.