Ah, okay. A 'B' might not be what you were hoping for but it won't prevent you doing Science A-Levels, especially if you smash Additional. What does your chosen sixth form ask for? Most only require a BB. Neither Oxford or Cambridge have specific GCSE requirements (unless you're studying medicine) so even if you couldn't study at your chosen sixth form you would still be able to study at either of them, assuming you get excellent A-Level grades. Don't panic if your school won't let you re-sit, just concentrate on getting a good grade in additional and you'll be fine.