Hi,
I will offer you the advice that I was given at GCSE. Study the most important exams (i.e. the ones that you will be taking at A-Level, or wish to pursue further) the most. Spend your designated amount of time on these subjects - trying to achieve the highest grade possible. However, try and leave maybe an hour or two a day to go over the lesser subjects. Whilst your focus will be mainly on your main subjects - you will still need to try and retain a grade at GCSE, because some colleges don't particularly like lower grades (like E + F's) even if you are not studying those subjects.
Why not try and make a revision timetable? Keep your focus on your important subjects - for example, your maths, English and history. But, slotting in the 'less important' subjects when you have time will make sure that you retain any grades that you currently have.
My best advise, as a student, is to do what you think is best for you. Have a talk to your chosen college, and see their minimum entry requirements, and base grades. Keep your head up, and I'm sure you'll do great!
Good luck in your exams, I hope that you do fab!
I hope that this has helped x