They may set some prep work as Muttley79 has noted, but I actually think you'd be better off expanding yourself academically in other ways. If you're aiming at top grades, you're probably aiming at top unis, and starting to read around subjects now (out of pure interest) will really help when you come to choose what you want to do and when you start making applications.
Out of interest what are you doing at A Level? Might be able to help with interesting topics/books/resources.
Also, I only started seriously studying in January Y13 and I did great so don't stress.