For what it’s worth (not very much), I don’t have the same reaction as my fellow parents. If you have a lot of talent (and verified achievement) in some significant academic area, and you have proved the ability to get top marks on your A-Levels in all subjects, I don’t think you would be rejected out of hand. You would need to find teachers to attest to your brilliance convincingly, and write impressive essays. Yes, it would be a long shot, but I don’t have the same sort of revulsion the others seems to have.
Gibby’s daughter’s transcript is completely irrelevant. She was a hard worker and an excellent performer at one of the most competitive high schools in the country. No one would have told her that she was not a good candidate for Yale. She didn’t have the benefit of A-levels to show her academic achievement on a standard scale, although given her particular school that hardly mattered.
You seem to have been raised in a system like that of most of the world, where internal grades are meaningless. Admissions officers, I think, are capable of understanding that. You don’t have to be doing all of the busywork and sucking up to the teachers all of the time if you are in fact learning what you have to learn and growing as a scholar, thinker, and doer, even if the norm for high achieving American high school students is never missing a quiz. If anything, you may well look like someone who needed more challenge than school was giving him or her, and if your extracurricular and quasi-curricular activities make it look like you weren’t just vegging out during the school year, you could be a perfectly viable candidate. That doesn’t mean you will be accepted, but I wouldn’t discourage you from applying.