Being a designer isn't about qualifications, it's about your work. You only really need decent qualifications to get into college/uni, and even then a degree won't guarantee you a job or a high quality portfolio of work.
Just try and do your best in all Maths, English (English language especially, as you'll be working with words a lot), and Science GCSEs to ensure you have a good foundation for further education, then just take what you think you will enjoy. If you have a graphic design module then that's obviously a bonus.
Like Paul said, ensure you study the foundation subjects of English, Maths and Sciences...these are essential regardless of want you want to do if you want to progress onto further study. If creative arts/design are available then, again, it goes without saying that you'd this this as an option. I would say however to study a range of subjects and NEVER put all your eggs in one basket, and please bear this in mind if/when you start thinking about A Levels. For example, if you do A Level Photography, Fine Art, Media and Graphics, along with a good folio, your chances of getting onto a graphic design course would be pretty strong. However, if you change your mind about careers at some point, and want to study something more academic, you're going to be pretty stuffed! You'll most likely have to study at Level 3 again. However, if you picked say Art, Maths, Biology and Physics (as an example), with a good folio, chances are you'd still get on the graphic design course anyway, but should you change your mind, you options are much wider...think about it.
I'm not dictating what you should or shouldn't do, but rather suggesting you be smart about your choices! Trust me I'm in a similar boat now! A few years after my graphic design degree, I'm considering applying for nursing or physiotherapy in the next year or two but have to do a science access course first because I chose all creative subjects at A Level!
@ Paul Murray...27 is NOT old!! Don't you DARE say that!! (I'm 27...I feel old enough without you saying that! Lol)
No worries I'll let you off lol. I know how you feel though. That feeling when you've handed in your last ever project...It's a mix of relief, tiredness, happiness that you made it, but also a sense of mourning/grief knowing it's the end, you won't see the tutors again and you're on your tod! Good luck for the future then and hope you get the grade you want. I'm personally looking forward to going back the classroom...I miss having assignments and deadlines in an academic sense.
Thanks for the replies and the advice! Those are interesting views as I had never thought about that before. Thankfully this year I've already picked English, Maths, Physics, History and Music which is a wide range of subjects. So next year I can pick Art, Graphics and Computing to focus more on art/design subjects : )