I first became aware of the two-finger, palm-in gesture being an insult in the UK from watching *The Young Ones *on MTV in the mid 80s (Vyvyan used it a lot). Although, doesn’t it always involve a distinct upwards motion when displaying? I thought that implied ‘up your nose’ (i.e. with both fingers) or something.
BTW, according to Naked Ape anthropologist Desmond Morris at least, the middle finger is an insult because it resembles a cock & balls. Same goes for why pointing your index finger with the others clenched is considered very rude & impolite.
Although it’s not widely used, this insulting motion is understood in the US, though here it would be read as “You’re a jerkoff” (‘wanker’ isn’t really known here).