You can have ADHD without having miserable grades. The issue is that boredom may mimic some of the disorder's symptoms, making it hard to distinguish what's what. Personally, the more I am challenged the better I feel. I would definitely apply to Harvard if I were you and perhaps some other top tier US universities, but I wouldn't rule out Oxbridge. Personally, I am not convinced that Harvard is more challenging than Oxbridge. It is very different, though, and it may suit you better.
The vast majority of applicants to top universities is very bright and has excellent credentials. That means a lot of excellent applicants are turned down due to a lack of capacity. Top grades, 20 A-levels and perfect SATs are not enough to get you admitted, because hundreds of applicants will have the exact same credentials. You need to stand out, have won medals in international olympiads, written books, published papers, won scholarships, to name a couple of possibilities. In fact the admissions process is much less transparent. They just have to like you, which doesn't make it any easier to be admitted. It still is quite a lottery.
Good luck!
By the way, I skipped the last year of secondary school (which is 12th grade here) because of extreme boredom.