I PMed this earlier to magna cum laude, and felt that I should post it here so everyone could benefit.
Truth be told, American admissions officers at different universities will assess international students in different ways, so it is hard to make generalizations. Once you narrow down a list of US colleges you are interested in transferring to, you need to check out the website and (really advise this) call and email them asking how they specifically assess applications from British transfer students. Someone else was PMing me, asking me for advice in applying to Yale, and apparently Yale doesn't require the SAT for British students and A Levels are fine. But I know this is not the case at the majority of US universities... you need to make sure you know exactly what tests each school (it's okay to call it this in the US) requires.
If you aren't sure yet entirely where you want to apply, I'd advise looking at the Princeton Review website (google it). You can sign up for free, and it'll give you nice descriptions of the colleges you are interested in, highlighting academics, night life, ect. For the most part, the descriptions are fairly accurate.
Ah, SATs, a royal pain in the ass, I might tell you. You can take either the ACT or the SAT, and it really depends on you in terms of which is "easier." Most students looking for entrance in top US colleges take the SAT which seems to be the more popular of the tests, but if you can score really high on the ACT, colleges will be equally impressed. I would advise you try to take the SAT, unless you know you will score a lot better on the ACT. In terms of scores, generally the rule of thumb for selective US colleges is that anything above 700 in each section is enough to pass the initial threshold to where colleges then look closely at your performance in high school and outside activities. There is not a guaranteed score that will gain you acceptance into elite colleges, unfortunately. Many students have been rejected with 800s (the top score) and just to give you an idea, I scored 690/710/750 (2150) and probably could not have gotten into an ivy or similar school. You should know all the material covered on the exam (A levels is more than enough) which is basic math, English reading analysis, grammar questions, and a timed 25 minute essay. However, keep in mind that the SAT can be difficult to score well on, despite the fact that the questions may be a joke compared to what you are used to in A-level. The math is easy, but was really a struggle for me because of the time limit and the fact that even a few stupid mistakes will dramatically bring down your score. Every question you get wrong on a section will majorly cost you . Most students in the US do some preparation before they take the SAT (well, the serious students anyway) and I’d advise that you don’t go into the test cold without knowing what to expect (the format of the questions, the time limit for each section, how questions are graded (you lose -1/4 of a point for each incorrect answer, 0 points for blank answers). You should definitely take a practice test and I’d advise ordering a prep book so you can learn some of the basic strategies and familiarize yourself with the material. If I were you, I’d order the SAT I prep book by Princeton Review in addition to the official College Board SAT book (both of which can be found on amazon.com). You’ll have to pay international shipping, but it is definitely worth it. If you have trouble finding what I am taking about on amazon, I can give further clarification later. As long as you pull a 2100+ or preferably 2200+, you should be fine. SAT scores are not everything in US admissions, and your A Levels will probably be more important. Also, plan ahead. A lot of students take the SAT more than once in order to get the best sub-scores they can in each section (colleges will combine results from multple tests for your total score). Also, if you totally screw up the first time, you'll have a chance to do more prep and redeem yourself.
Hope this is helpful.
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Oh, and I forgot. To the original poster: don't worry about your score on the practice test too much right now. It'll go up (usually you get smarter somehow later in your academic career when you sit down to actually take the test) and it is very possible to bring up your score if you prep diligently. That is perfectly decent score given where you are at right now, don't worry.