The entry criteria aren't based on something like the SAT, although International Baccelaureate perhaps comes close in Europe? Not sure, maybe someone else can explain?
So there are no prep classes or anything like that in the UK. Everyone sits the same set of exams (modulo being Scottish or English/Welsh/NI, which have different types of qualification) and those results dictate whether you can go to University or not. I suppose you could study more if you wanted?
Grades only really matter for medicine, psychology, and perhaps law. If you want to be an engineer or physicist or mathematician etc, they take everyone.
So there are no prep classes or anything like that in the UK. Everyone sits the same set of exams (modulo being Scottish or English/Welsh/NI, which have different types of qualification) and those results dictate whether you can go to University or not. I suppose you could study more if you wanted?