Hey, CCers. I am a junior in an American high school right now, and I am thinking about applying to universities in the UK. I would be interested in London, specifically. How do I know if it’s right to consider them? I guess it’s easy, in theory, to say that I want to apply to them, but how do I know if it is a good fit for me? I am applying to Canadian and American universities, as well. Which school in London would be the best for an American student? I am hoping to study government or economics. I was thinking King’s College and London School of Economics if I were to apply. Does this seem like a good route? I absolutely love London, and I think it would be a once-in-a-lifetime opportunity. My only fear is that something just goes completely wrong, and I won’t have anyone there for me in London.
there are things to fear that are more likely to happen than some random thing going wrong 
The UK can be a great experience- if it suits you. If you just want to hang out in London, do study abroad. Full time is a totally different thing. Some key things to consider:
Standardized testing is the key to admission; APs in *relevant subjects are crucial (KCL will also accept subject tests, but LSE does not). ECs, except those that demonstrate interest and ability in your chosen subject, are irrelevant. GPA is rarely considered, and when it is considered it is usually just a qualifying #.
*You apply to study 1 subject (or specific subject pairs/groups). Most courses are 3 years, have few if any choices in the first year, and only choices within the subject(s) after that.
*Every UK college has extremely detailed info on their websites. The standard offer will be listed as a set of 3 letters (eg, AAA), which are the scores needed for admission to the course. You can cross-check the section on international qualifications to see what is required (LSE wants 5 APs with scores of 5; for politics they are looking for subjects such as History, Gov, English; Econ; Languages; Math). The websites also list what you take each year, how assessments are done, etc.
*In general, assessment is based on final exams; there is little or no weight on continuing assessment (ie, tests or quizzes). You need to be pretty good about staying on top of your work on your own.
*Students are expected to be very independent- there is little or no handholding, even for international students.
*There is no meaningful financial aid; expect costs of $40K+
If you are still keen after doing some more research come on back- there are a bunch of well-informed UK posters who hang out around here!
Just to add to @collegemom’s helpful summary, if you decide on applying for econ one of your AP scores of 5 must be in Calc BC. It’s a very math-heavy course.
Look carefully at the course descriptions on the websites to see what suits you.
In addition, I’d suggest considering studying outside of London (lots of excellent universities within 1-2 hours by train) rather than IN London. As a student, you have all the downsides (especially cost) and not many of the advantages (because you don’t have enough money to enjoy them). There are no “campuses” per se, students are scattered throughout the city. I find it preferable to attend, say, Warwick or Durham and spend half term (=nice long breaks) in London.
That being said, when you see a requirement of A level Maths, they mean a 5 in Calc BC as a minimum (A level maths goes further than Calc BC).
I refer to English undergrad as “grad-school-lite” because in a good number of ways, it’s closer to a grad school experience than a typical American college experience.
If you just love London and especially if you’re not dead certain about what you will major in, you should know that pretty much every American college these days offers a lot of study-abroad programs.