It is, via the interview. This is why A-level results are a coarse filter, and why they have different standards for state vs private schools; state school kids with 3 A's presumably excel in the interview to the same extent as private school kids with 3 A*'s.

It cannot be understated how much of an advantage someone who went to a private school has over someone from a state school, with respect to the entire process (exams/admissions tests/interview prep).