make sure you get really good grades in most of your exams along y12 as it helps to maintain good predicted grades.
I guess it also depends what you want to go into at the uni, as you wish to do maths, I’d say you’re doing pretty good so far. Just make sure when it gets to writing your personal statement, or whatever the US uni would like, you show don’t tell. What I mean by this is, instead of saying you did experiences a,b,c.. say how your interest in maths lead you to do a which lead you to do b then c and so forth
I think that was the best advice given to me when writing my personal statement which allowed me to get offers into the top London unis.
ALSOOO: download LinkedIn. It’s amazing trust me, you can speak with other students in your position but also students who are in USA unis who also were from the UK. Speaking with them can help you see what you can do further but also they give pretty good advice. I had a couple uni students read over my personal statement and they gave me amazing feedback.
Good luck on this journey!