The average Brit/Irish is not anti-American. What you read is from the vocal leftist minority and leftwing media who dislike America for political reasons.
It's not from reading, FRiend. It's from unpleasant personal experiences on three different U.K. visits. What you say is true speaking of "averages." Unfortunately it's the folks on the average's "downside" who've poisoned my travel experiences there. It's the totally unprovoked verbal attacks simply because one is American that have convinced me to spend my travel dollars in friendlier places.
What's sad is I was always pretty much an Anglophile -- until I experienced today's reality. I don't go looking for trouble. My wife and I try our best to blend in and observe local customs. But there's an outspoken and sneering anger by some toward Americans that I have a choice to avoid in the future, and avoid it I will.