You will need to declare all your qualifications. However grades and the SAT are only one side of the US college application process, and are largely just a tick box exercise; everyone applying to those colleges has top grades in pretty much everything. Excellent grades are a necessary but not sufficient criteria for getting into an "elite" college; bad grades can fail the necessity criteria and thus result in you getting rejected, but excellent grades are no guarantee you will get an offer. The much more important side for those "elite" colleges is your extracurriculars, which unlike for many UK unis are very important in the US.
Those kinds of colleges usually frame their admissions in terms of 'you have excellent grades, that's a given - now what will make you a unique and active member of our college "community"?'. As a result they look for extensive evidence of extracurricular involvement, which is both long term (i.e. not stuff you started just a couple months before applying) and at a high level (i.e. necessarily being in leadership positions, and for competitive things competing at probably a county or national level at least. Applicants may well have been featured in their regional or possibly national news for their initiatives they undertake in their leadership positions). They want to see you have done more than just turn up on time to your after school club, but have actively participated in it and developed it to leave a lasting (positive) mark.
Basically if you are likely to be on track to be a strong applicant for Oxbridge, LSE, UCL, etc, in the UK, and have a range of very notable extracurricular activities, then the US is a likely option. Having poor grades overall, not having done anything except sit exams from year 11 to year 13, doing poorly in the SAT/ACT, all of these things would likely make you a non-competitive applicant. Unless, of course, you're a "legacy" applicant i.e. your parent(s) went to the college in question (and are rich and have donated as alumni)...which is still the case and still happens in the US (I think there was some statistic that like 40% of Harvard admissions were legacies...).