You are absolutely not too late, you still have a few months leeway. I’m not sure what a UK student would need to apply for colleges in the US, but you’re in luck because most colleges have marked the SAT as optional for students set to graduate in 2021 (due to coronavirus). However, arguably the most important part of an American application is extracurriculars (sports, awards, instruments, etc). Even if you have absolutely perfect grades, if you want to go to a good university, you still need ECs. When you apply, you will have the opportunity to list them all. The American application process is extremely tedious and expensive, be prepared to write an essay for every school you apply to and potentially an interview. You may have the opportunity to apply early decision. When you early decision a university, it means that you will be more likely to get in, and will receive your acceptance letter earlier, however, in exchange, you must attend that university if they accept you. My advice to you is that while it is still summer, do extracurriculars. Get a job, become an intern, volunteer at old folks homes, compete for awards in things your good at, otherwise you probably won’t get into a high tier college.