Unfortunately Harvard are one of the few American Colleges which look at your GCSE’s and A levels.
It is suggested that whilst A levels determine whether you get into Oxbridge a minimum of 6 A*‘s at GCSE is required for all private school students.
Harvard is certainly close to, if not at, the level of Cambridge and Oxford and so with your lower GCSE results it is vital that you obtain excellent SAT scores.
This is not to say you cannot gain acceptance to Harvard. American Colleges’ more holistic approach to admissions will give you an opportunity to ‘balance the ticket’ with your extra curricula activities and character.
Good luck!