I live in leafy Oxfordshire in England and was thinking about going to the USA for University, is there anything I need to know (the most I know about America is prejudices and bad sitcoms).
You know, it seems like I'm so qualified to answer this question that I should be able to do so while standing on my head—I once wrote for a college guidebook company, I've lived or studied at three American universities, I spent a year abroad at a U.K. university, and I have a very large head (making it easy to balance on). But it's actually kind of tough, because every university is so different that I can't tell you exactly what to expect. Comparing Vassar to Penn State is like comparing a poetry class to a poetry class that is situated within a prison riot. Large state schools and small, private liberal arts colleges are so unlike each other that the only thing they have in common is rooms in which learning allegedly happens.
That said, there are some generalizations I can make about how the experience is different between our two countries. I'm sure you know the obvious cultural stuff (what you call "American football," we call "football;" what you call "football," we call "booooo!") so I'm just going to stick to college-related stuff. (There's your first difference: anytime you'd say "going to University," we say "going to college," even if the college is at a university, because I don't know, we just do.)
The drinking age is 18 in the U.K. and 21 in the U.S., which makes for a pretty dramatic difference. At my university orientation in the U.K., a policeman got up on stage and told us that, when we became impossibly drunk ("when," not "if"), we should not decide to go swimming in the ocean during a storm, which of course tons of people did anyway. When we met the faculty and staff, they offered us wine. The student union had a bar in it. This was all completely alien to me, because alcohol is a much more clandestine thing on most U.S. campuses—by the time you're legally allowed to drink, you're probably graduating. Obviously it's a thing that happens anyway, but if I had to pick the single biggest difference, this would be it. In the U.K., you can sit down at a pub with your professor and have a drink, and he doesn't go "PTOOO!" and spit-take his shandy all over the place while shattering his monocle in surprise.
Fraternities and sororities are a big part of campus life. There really is no U.K. equivalent to this kind of social club. Maybe if you formed exclusive cliques based on which football club you supported, and developed a bunch of rituals and rites of passage, and had your own specific chants, and uh... okay I guess there's an exact U.K. equivalent. But frats and sororities have a more direct influence on campus social life than anything I noticed in Britain. Again, though, this varies dramatically from school to school—participation literally ranges from 98% to 0%. You may have frats everywhere and you may never even see one.
America is like a jillion miles across and a jillion dollars in, uh... money-having. A lot of our schools have ridiculous wealth compared to U.K. schools, and most have a lot more space. As a result, everything tends to be bigger, more spread-out, and full of expensive buildings that look like a robot melted while trying to conquer a convention center.
Befitting our largesse, many schools have space for a huge stadium, a budget to have kids play football in it (not the kind you like; the kind you don't), and the school spirit to compel tens of thousands of students to show up for every game, even if their mascot is just a tree that is insane.
I've been told our undergraduate study is like your A Levels, but I've never been a British teenager (as far as I know), so I'm not sure. Your university experience tends to be a lot more focused, though, while ours is pretty broad; most of your classes branch out from the same topics, while ours are all over the place. Also, if you apply to a U.S. school, you'll probably have to take the SATs, which is when you sit in a room and struggle to remember all of the math that you you lied about understanding in the first place.
The differences will depend on where you wind up; a huge state school with 50,000 students will feel more foreign to you than a small private school will. Honestly, though, as someone who's been through both, the experience is pretty similar between the two countries. You arrive at your cramped, tiny dorm room and buy either a Godfather/Scarface poster or an Audrey Hepburn poster, depending on your gender. You find out that you're allowed to eat cafeteria hamburgers all day, and you do, even though they are gross, and suddenly cannot fit in your pants. You are told to read "Bartleby the Scrivener," and don't, because of video games or laziness. Look up the schools that interest you, pick one whose size doesn't frighten you, and you can worry about how we spell "aluminum" later.