I dont think you should worry too much about your grades, yes at your age if you want to get sponsored you may need better grades and i think they like them in Physics etc.. But believe me there are plenty of people myself included who dont have any A' levels at all, however I'm 30 so will be self sponsoring myself on a modular route, I think it will matter less as you get older. At the end of the day I dont see it stopping you if you really want to do it, and the most important thing will be how you come across at an interview and how you fly! Who would you want flying you, someone with staight A's in their A Levels or someone who was a good pilot!
Also you have plenty have time to decide, just dont wait till you're 30.
Best of luck
Dan