Agreeing with Conformist 1688 & Alfonsia, and adding that admissions is very much based on standardized testing, SAT / AP / SATII. With the exception of St Andrews in Scotland, ECs are only relevant if they are directly related to the subject that you are applying to study, and GPA is a negligible factor.
Every UK university lists every course that they offer, the requirements for admission (what test scores they require, and if relevant, the specific subject(s) required); a thorough description of the course, usually down to the exact modules you will take in each year; and information for international students. Typically English courses are 3 years (except for modern languages and many sciences, which are often 4); Scottish courses are typically 4.