They are VERY different! I attended a UK uni for a year (I'll keep the name of it confidential), and I think that it's very difficult if you aren't certain what your major is yet. Going to a UK university is basically putting yourself on a very narrow track, which is not necessarily a good thing. You must KNOW what you want to do with your life when you go in, so I would highly discourage American high school students to attend a UK university their freshman year, because high school simply does not prepare kids for knowing which subject they want to major in.
Or if they do know, they might discover they don't like the particular way the subject is being taught. I knew what I wanted to be doing (psychology), but realised a few months into it that they were focusing mainly on statistics and the practical side, which was not the area of psychology I was interested in focusing on. I would have continued three years learning an area of the subject I didn't like, without being able to learn other areas. In other words, the courses are less specifically tailored to what you want. I also took sociology as part of my joint-degree program, but I really wanted to fit in visual arts as well, a huge interest of mine as an artist --- but unfortunately, I couldn't.
Furthermore, people at the uni, like the tutors, were generally UNHELPFUL and unsympathetic to my needs as an international student, the same sort of general unhelpfulness I found to be prevalent in most of the UK. It was very difficult.
And trust me, trying to change majors if the class isn't working out for you is much more hassle than it's worth! It's not just changing classes, it's actually changing majors! If you've been in the American system, be prepared for a shock at the inflexibility of it. The positive side is you only take one or two classes and get less class time, but the negative is that you don't get as well-rounded an education at all. Plus, there are lots of essays and fewer exams, which means that cumulative exams count for a huge percentage of your grade.
The system is FAR stricter. If you are sick on the day of an exam without giving a notice the day before, for example, you could automatically fail the course! And what happens if you fail? You will have to start over as a fresher again the next year. No questions asked. :'(
UK universities might be okay for graduate students or upper classmen who know
EXACTLY what they'll be doing, or for study abroad students, but they're definitely not for someone who wants to discover themselves a bit and try different areas of study before they really know what they'll be doing for good.