As a Brit, I found this post rude:
I flagged the post and the flag was declined, which is fair enough, but I'd like to know what degree of insulting towards the British is acceptable light-hearted banter and what is considered to cross the line.
The insult in the wording of the post is subtle ("In the USA we're rational" — i.e. in Britain they are not rational) but the context shows a pattern of intention to rile — shown by the OP accepting a heavily downvoted answer and the following comment below that answer:
I find this explanation to be the most convincing thus far; and, after seeing Life of Brian and other perverse films from this region I'm inclined to think it is inline with their culture. Good job doing the research required.