The Freedom of Information Act 2000 specifically exempt certain types of information (as taken from the HMSO website) that is, or may prejudice, the following:
Information accessible to applicant by other means (section 21)
Information intended for future publication (s22)
Information supplied by, or relating to, bodies dealing with security matters (s23)
National security (s24)
Certificates under exemption 3 and 4 above (s25)
Defence (s26)
International relations (s27)
Relations within the United Kingdom (s28)
The economy (s30)
Investigations and proceedings conducted by public authorities (s30)
Law enforcement (s31)
Court records (s32)
Audit functions (s33)
Parliamentary privilege (s34)
Formulation of government policy (s35)
Prejudice to effective conduct of public affairs (s36)
Communications with Her Majesty and honours (s37)
Health and safety (s38)
Environmental information (s39)
Personal information (s40) - see the Data Protection Act 1998
Information provided in confidence (s41)
Legal professional privilege (s42)
Commercial interests (s43)
According to section 44 of the Act:
"Information is exempt information if its disclosure (otherwise under this Act) by the public authority holding it:
is prohibited by or under any enactments,
is incompatible with any Community obligation, or
would constitute or be punishable as a contempt of court"