Timely question, today being Results Day. (TV coverage:
https://news.sky.com/story/a-levels...sity-as-thousands-await-exam-results-12675608)
Completely different exam model than the US exams, and MUCH more high-stakes. For example, there is only one re-take allowed per year, and if you want to do a re-take, you also have to re-enroll in classwork of some kind to be eligible, so unless someone had a really unusual situation on the original exam date that caused a meltdown scenario, doing a re-take is pretty rare. (You can appeal results, though; some of the scoring is subjective, and they will review for possible score changes.) The entire country takes the exams on the same day, and get results on the same day, so Results Day is an official Big Deal.
The UCAS system determines university admissions throughout the UK, and most of the premier schools will not place you without a certain A-level score in your preferred field of study. (For US applicants, they normally require a certain minimum score on the SAT, plus scores of 4+ on at least 3 AP exams; you are only eligible for a "place" in one of the AP subjects.) UCAS:
https://www.ucas.com/