PACE and the Codes of Practice

The Act, the codes, and Code C on detention and questioning.

Police and Criminal Evidence Act 1984
The Police and Criminal Evidence Act 1984 on legislation.gov.uk. The Act makes further provision about police powers and duties, people in police detention, and criminal evidence.
PACE codes of practice
GOV.UK page for the PACE codes of practice. It says the codes regulate police powers and protect public rights.
PACE Code C 2019
PACE Code C 2019. Requirements for the detention, treatment and questioning of suspects not related to terrorism in police custody.