Hi, I am a student from El Salvador, and I attend an American School. When I graduate, I will receive a USA High School Diploma and I will have taken 9 AP classes (all with scores of at least 4) and the SAT (with a score of 1900). Also, I will soon take the IELTS and TOEFL.
I am struggling with understanding the UK system of requirements and I would like to know more about selectivity. I am interested in Edinburgh and Glasgow in Scotland, and Manchester in England. What are my chances of getting into these schools? How can I begin to compare them to American schools, with the system I am used to? The course I would take is Psychology.
I want to study in the UK because the US has me disinterested with it’s culture and general attitude towards latinos in recent years. I would much rather study in a more cosmopolitan, European city, and explore different world attitudes. Also, tuition prices are WAY more affordable.
Each university has a webpage with requirements for american students (AP results and SATs mostly). Look it up.
You have good chances at the unis you listed. Go to UCAS and print out the section for guidance counsellors, it explains how to write a letter of rec. Head over to studentroom and read about writing personal statement.
I imagine you are graduating from escuela americana? You should be well prepared academically and I think you have good chances at schools that are better than Glasgow and Manchester, especially if you raise your SAT 50-100 points. Good luck and let me know if you have additional questions. I just went through this process with my daughter (btw, she spent a year at escuela americana in 6 grade 
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Thanks for your reply! The UCAS section is definitely helpful. And yes, I am graduating from EA hahaha. May I ask where your daughter is going?
Her two top choices are UCL and Edinburgh.
If you go for pure psychology, make sure you have AP calc exam or at least Math 2 SAT. Each uni will tell you what subjects are required for certain courses (you will see that within British qualifications “required subjects and grades” by course). A and A* are equivalent to a 5 on AP exam.