Not everyone is automatically entitled to see the data which is held on them. There are certain circumstances under which data controllers are exempt from providing information to data subjects.
National Security - If required for the purpose of safeguarding national security, data controllers do not have to disclose the data held to data subjects.
Crime - Data which is being held in order to prevent or detect a crime does not have to be disclosed.
Taxation - any data collected for taxation purposes is exempt
Health, Education and Social Work - personal data about the physical or mental health of the data subject. You have no automatic right to view your medical records. If a doctor believes that viewing them would harm your mental well being, they can be withheld from you.
Schools and examinations - personal data relating to the present or past pupils of a school. Examination scripts and examination marks held by examination authorities
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Data Protection Act Exemptions