The DPA and FOI excludes anything that is "sensitive". You also need to think about other police forces as the police are a federated bunch and while you might get a PNC check you won't get anything that other forces have about you.
Not necessarily true as a subject access disclosure deals specifically with convictions that are unspent and all police stations have the same access for this. Only criminal intelligence is held regionally which is not disclosable on a subject access request.
The police Federation is a membership for officers that is to do with their welfare and is nothing to do with disclosures etc.
Any organisation has the legal right to have an employment risk policy that specifically excludes people with criminal convictions that are unspent, irrespective of how minor the conviction was and how long ago it occurred. That's their right whether you like it or not.
The Home Office is the right place to begin your search. Good luck
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