What were your actual marks in prelims?
So city campus, big enough that nobody knows anything in particular about you, and good for econ?
Chicago is the obvious place, Columbia would be, but it’s a tough place to move to, b/c of the Core curriculum requirements. Georgetown might be too small for you. UCLA or UCB would do fit. Johns Hopkins could work.
It sounds as if you are going back for Michalmas term, in which case I suggest trying to go back with an open mind. Some of the things that you mention are things that can be interpreted other ways. For example, “They keep saying Trump will be re-elected, which is just code to say Americans are stupid and backwards” is (for many Brits anyway) not code but an assessment that rather a lot of Americans (both pro- and anti-) also have made.
“These people act like they know everything in the world, and seem to think they know more about US politics than I do” is certainly true for a lot of English people (and especially 18 year old PPE students). However, if you move back to the US and talk to the international students at your new top-tier uni you will discover that is the impression a lot of them have of Americans as well. Ditto the tendency to self-segregate.
Becoming a minority- which, as an American at Oxford you are- takes some adjusting. You have to learn to look at things from a whole different perspective.
It’s a pity you didn’t make any friends in your first year staircase, as that is where a lot of people get their base. Given that you are in PPE you are in at the deep end of politically opinionated people, who all want to be Prime Minister some day. If you are involved in the Oxford Union, that is an even deeper pool of private school / ambitious / competitive people- and hand on heart is not representative of the student body. And if you are at ChCh double up on the above!
There are other people at Oxford, and if you go back this autumn look for them. If you got your fingers burned at the Union know that you are not the first to have that happen, and there is life afterwards. Get involved in something else- check out RAG, for example, which has a lot of nice people in it.