Need honest advice

Top = top 10 or top 50? It really depends on your supercurriculars and what subject you're applying for. Not having extracurriculars would put you at a disadvantage, but strong supercurricular accomplishments could compensate. (And either way, like en8827 said, luck is a huge component.)
Also, unless you're in year 13 already, it's not too late to start an extracurricular.

the US application system is almost completely down to luck. I have all As and A*s at alevel, all 7/8/9s at gcse, and both extra and supercurriculars but i still only got into my safety school there. Having only supercurriculars might not be ideal but its definitely better than nothing. If you can write some really good essays then you definitely have a shot.
(plus as you'd be paying overseas fees that also helps your application because that's all they really care about)
As I said, its mostly just luck getting into the top schools, so there's no harm in trying!

Top = top 10, I want to apply for computer science. I’m in year 12 and I do 4 a-levels predicted at A or A*. I think I have some strong supercurriculars but for my extracurriculars I don’t really have anything special

Top 10 unis really pay attention to your essays - i would even say more than your ECAs. as most of them require additional writing supplements, its important to showcase your creativity and passion for what youre studying. Unlike the ucas personal statement, don't be afraid to get emotive or informal - if you can show you have a personal connection to the subject (and university) it really goes a long way. Good luck!