gcse's are nice but by no means essential your school easily makes up for your slighlty below average grades, An academic secretary mentioned to me that most courses at cam now get significantly more applicants with 8A* or above than there are places on the course, but obviously this is not how they decide place i have met people with 1-11 A*'s while here!) You should definatley be applying to cambridge, provided you do well on your AS (I would consider dropping bio or chem though as you will want really high maths marks if that is your subject and doing all 3 sciences could be tough, but its up to you and if you think you can handle it go for it)