The Information Commissioner’s Office has published useful guidance on what constitutes personal data for the purposes of the Data Protection Act.
The technical guidance note - “Determining what is personal data” - is designed to help data practitioners decide, in difficult cases, whether data falls within the definition. If it does not, the rules of good information practice imposed by the Act do not apply.
This is particularly important to public authorities responding to access requests under the Freedom of Information Act. If the information requested is personal to the requester, the Act requires the request to be handled as a subject access under the Data Protection Act.
If the information requested is personal data about someone else, the Act states that it should not be released if it would involve a breach of data protection rules.