UK unis admit primarily on test scores- APs and/or SAT subject tests in subjects relevant to what you are studying. GPA gets little if any attention.
You are admitted directly to your ‘major’ and generally only study that subject (or subjects, if you are taking a joint course). You rarely take classes outside of that subject (there is some variation in Scotland, but not a lot).
Courses are highly prescriptive, and you will have few (if any) class choices in the first year or two, and will have a relatively short list of options after that.
Class size will depend in part on what you are studying, but in most cases you can expect relatively large lecture classes in the early years- as you would a state university in the US.
UK unis are much more hands-off than in the US. You get much, much less support from the administration and the professors: you are meant to be an adult and mind yourself.
The students tend to be not just friendly but very supportive of each other (b/c they need each other!), but most unis are not campus-centric / school-spirited in the way that they are in the US. Although there is a lot of variation, the proportion of students who live off-campus / not in campus housing is generally larger than in the US ( though there is almost always housing for international students).
So, IF you have a subject that you love enough to be what you study full time AND you have at least 3 APs/SAT subject tests in relevant subjects with scores of 5s/650-700+, some unis that might be interesting to you are:
St Andrews: in a teeny tiny town in far Scotland, it has fun traditions and a pretty tight student body. They actively recruit US students, so they understand what Americans expect, and there is big group of fellow Americans (as well as a large international contingent). Compared to most UK unis there is some curriculum flexibility. Although accommodation is only provided for first year (so you have to find your own in town for years 2-4), the system is well set up and manageable. You can apply through the Common App or through UCAS (the UK version)
Durham: in a smallish town in the middle of England, it is a ‘collegiate’ university, meaning you belong to a college within the university, which gives you a built-in community. Durham is more flexible than many unis, and most courses are 3 years (except modern languages, which are 4, including a year abroad and sciences where if you do a 4th year you finish with a Masters). Top notch academically and relatively smaller classes.
Oxford or Cambridge: application deadline is 15 October, and most courses have an aptitude test that you have to register for by then, so it would be a push. Even more than the others it is strongly recommended that you really, really like the subject that you apply to study- it is very, very intense and very, very hard to change if you find you don’t like it. But it is the ne plus ultra for teacher involvement- you meet your professor(s) for tutorials (Oxford) or supervisions (Cambridge) at least weekly in groups of usually 1-3 people.