If you miss an exam for illness (for which you must get a doctor's note and present it at the very latest, the day after the examination), then your school cannot charge you. You will likely recieve a grade based on previous examinations for that subject, or your teachers will have to provide evidence (i.e. mock exams) for which grade your should recieve.
However, if you miss an exam for any other reason, then you will be held responsible. You will got a 0 (U) for the exam, and it depends on your school's policy on whether or not you have to pay for the exam.
No one here can say, it completely depends on your school.
I can tell you that for my college, if you:
1) Have <80% attendance for that subject.
2) Fail to achieve an E in coursework/practical assessments.
3) Fail to contribute or show any basic understanding of the subject.
4) Miss the exam.
You have to pay for the exam.
However, at my school where I did GCSEs, we weren't expected to pay for the exam if we missed it.