It's true that the general opinion in England of Americans is not a good one, but in my experience it's based on meeting a very small proportion of your society (14 year old's who've just learned how to swear over the internet etc.). However on every one of the numbers of trips I've made to America I've always felt that as a whole, Americans are unfairly labeled, in fact in some areas Britain could take some lessons (customer service is considered a chore and something to be barely attempted in England!). But people will believe what they want and jump to false conclusions. Although admittedly G.W.Bush did little to further your cause
As for the American civil war, my generation (early 90's) seem to care very little about our history and culture as English people (unless they're playing "who's the better country" with other idiots on youtube, or trying to annoy the scottish boy in school), which is sad considering how much England has affected the world - albeit sometimes negatively - over the 1700 or so years since we were first documented. That's the key difference (in my opinion) between American and English people, you tend to be passionate about your history and achievements, we don't (on the whole) remember much that we are proud of! Our independence from the likes of the Romans came and went a long time ago, so things still fresh in American minds is taken for granted and wholly uncelebrated in England.
Maybe then it's not that we think little of you, but simply that, as a people, we no longer understand your passion for things which we've not had to think about, let alone fight for, in over 1000 years and take your passion as bragging, instead of a celebration of your still relatively new nationality.
Or maybe it's just that we seem to mainly meet the minority of idiots that you have and vice versa
I hope you all have had (are having? don't know the time differences :P) a good 4th of July! Best wishes from across the pond