Lots of schools only look at your top 6 to 8 exam results or take an average - or just look at the subjects you're taking for A Level. For example, my local grammar school takes an average of your 8 top exam marks and if it is a 7 or higher then you get in (which is definitely achievable for you).
It does vary from school to school, but as long as you get good grades in the subjects you want to take for A level, then it should be alright.
Good luck, and remember that you have plenty of time to work on your marks anyway