Resitting exams during Gap Year

Got 2/3 of my A-Level results, and they were not that good in my opinion:

Physics - B (73)
Chemistry - C (69)

I'm determined to get higher results if i manage to convince my parents to let me do a resit during my gap year. I've obviously never done this before, and even with the little insight i have on the situation, information on how to go about all this would be highly appreciated. Alongside that, the exams i would most likely be resitting are CIE A-Level Chemistry and Physics. Considering that for International students (excluding india) the examination periods for CIE are October/November and May/June, what would you say is the most optimal time to resit? Thanks to anyone who replies. Among all this i would still love to do some preliminary studies related to my course.