Can I request for an exam not to be marked after I've sat it?

No. They will not entertain you. That's why I don't think A levels are for most people. I would 1 million% prefer the US way of evaluating student. UK way is too outdated.

Can I ask how you know that for sure? Thank you!

No. They will not entertain you. That's why I don't think A levels are for most people. I would 1 million% prefer the US way of evaluating student. UK way is too outdated.

How do the US do it?

It varies from colleges to colleges. In general, U.S. colleges assess student performance through attendance (yes, attendance counts!), regular quizzes, presentations, participation, and several exams in each subject you take. No one major exam determines your future. I have friends who did their degree in the US. They said it is super easy to get top grades and they really enjoyed studying there.

I don't have black and white proof from the exam boards, but it's a very logical way of thinking. If such a request is acceptable, many may also ask to "revoke" their ungraded or poor grades, as if they had never taken the test. Such a request also undermines the authority and authenticity of the exam board. I would encourage you to give it a try and can't wait for you to prove me wrong.