Yes, I've ranted about this before, it's so irritating. Honestly, I have met waaaay more empty headed people here than I ever did in America. Some people here can't even tell me where other parts of England are (from London) even Essex! And whenever I mention something mildly academic like James Joyce, Mid-century modern, oedipus complex or use "big" words like cathartic, i get blank stares and no one know what I am talking about. And just listen to almost any conversation on the train and tell me how it's so much more intellectually stimulating than what an American would say. Even a British friend of mine said he really struggles to have any sort of stimulating conversation with anyone that goes beyond the weather.
Don't get me wrong, sure there is dumb Americans but there are dumb people here too. To me Boris Johnson is almost as bad a joke as some of our politicians. And listening to people rant about immigration is just like listening to Fox News.
I think Americans are sold the notion via shows like Downton Abby that Brits are prim, proper and smart but of course its not the case, just like the rest of the world is sold the image of "all" americans being fat, lazy and dumb (even though we are the most powerful country in the world.)
But yeah, what can you say? All I do is laugh it off. If people are going to think that way, nothing i say is going to change their mind. But i assume a lot of it stems of insecurity or our politics (which is fair, but also, Britain isn't exactly innocent either)