Aim for Oxford/Cambridge before you aim for Harvard (in my opinion) unless you are amazing at one thing (i.e. a sport) because Harvard (and other top us unis) love that (more than Oxbridge).
However, if you can’t achieve very good GCSEs, the chances of you getting 3-5 A/A*s (you realistically need 4/5 A*s for Harvard) at the advanced level is quite low (so the chances of Oxbridge AND Harvard are quite low).
Not saying it can’t be done I got 3 A*s and 1 A in my A levels after averaging a 6.4 in my GCSEs (partly due to my teacher-assessed GCSE grades in 2021 but that’s a separate story).
Aim for very good GCSEs (all 7-9) in your year 11 mocks, and then think about Oxbridge and the Ivy League.
I'm saying after the year 11 mocks rather than later (like GCSE results day) because you need to prepare very early to have a chance at admission to Harvard. After all, you need to do volunteering, extracurriculars, supercurriculars, competitions, etc.