The selection process is holistic, they consider everything together to build up a picture of the type of applicant you are. I would still say very roughly this is the order of importance: Grades (Mainly AS and A2 predicted, GCSE less so), Interview, Contextual factors, Admissions test, Written work, Personal statement/References.
Btw Cambridge doesn't use raw marks when comparing to other candidates, they just use it to see if it's likely you'll achieve your predicted grades.