More important is determining why you want to study for undergrad in the US and why “top 20” schools (which would be a disparate collection of schools that could differ a lot.
Also, as you’re probably aware, meeting minimum requirements (or even blowing past them) isn’t enough for Oxbridge, though as their admissions is much more like grad school admissions (to an American), you have a good shot if you are seen to have the potential to be stellar academically in the subject(s) you choose to study.