There are plenty of different universities out there. Some take high grades, some take low grades. Maybe it would be worth while looking at some universities you'd like to go to, if you dont get into your first and second choice. Have a list handy. When it comes to results day, if you dont get the grades you need just phone up and ask if you can go to the other uni.
If you haven't applied to UCAS yet then dont worry about the grades. Your teacher (the person who will write the statement on the back of the ucas form) will put down your expected grades, not necessarily the grades you are achieving at the moment. Why dont you arrange a meeting with them to discuss your grades and what you think you will get? Then you can look at the uni's which correspond with your predicted grades.
Dont worry about it too much. I didnt get the grades I needed for my choices so I went through clearing in the end. I ended up in Hull which is a really nice uni. I enjoy it here and probably do better as there is less competition among the students than if I had gone to a uni that expected a higher standard.
Good luck with your exams! Stick with the uni thing. If everything goes horribly wrong, you can always try again next year.
Thanks Helen, you have helped me see things more clearly.