The case of R (Application of the Centre for Advice on Individual Rights in Europe) and (1) Sec State Home Dept (2) Commissioner of Police of the Metropolis  EWCA 2837 (Civ) holds that police officers have the power to do anything that an ordinary citizen can do.
The Claimant contended that the police could not lawfully ask arrested persons about their immigration status in order to provide this information to the Home Office because a police officer did not have the same power as a member of the public to approach and ask someone questions on any topic for any purpose. Rather, the Claimant argued, a police officer’s powers had to be used to fulfil their duties and obligations as a police officer.
There was, however, the seeming concession by the Claimant that a police officer could question members of the public for non-policing purposes provided that they did not purport to exercise police powers or appear to be doing so.
The Court of Appeal rejected that entirely for the following reasons:
i. As a matter of capacity, a police officer had the power to anything that an ordinary citizen could do, including non-coercive questioning of a person in custody. They might, as a matter of vires, do anything that a natural person could do without the use of coercive powers, including asking questions that a member of the public could lawfully ask. The additional powers of a police officer to perform actions that would otherwise be unlawful for an ordinary citizen did not constrain or restrict their powers and rights as ordinary citizens ;p>
ii. These powers were subject to the constraints of public law; officers had to act reasonably and in good faith and in accordance with any other public law duties. What they did not have to do was find some specific police power to enable them to do something that ordinary citizens could do . This followed Collins v Wilcock  1 WLR 1172, where the court stated at 1178E that a police officer had his rights as a citizen, as well as his duties as a policeman .
iii. Regardless, the officers’ actions were for a policing purpose. There was no exhaustive definition of what a ‘police purpose’ was. On the facts here, the court was content to hold that the questioning of detainees on their immigration status fell within it .
The most interesting point is the affirmation of the principle that a police officer has the power to do anything that a public citizen can. This is a contentious point when it comes to the power of the Crown and government in other contexts – see, in very different circumstances, R (New College) v Home Secretary  UKSC 51;  1 WLR 2358 at : “it is open to question whether the analogy with a natural person is really apt in the case of public or governmental action, as opposed to purely managerial acts of a kind that any natural person could do, such as making contracts, acquiring or disposing of property, hiring and firing staff and the like…”
It could be said that it is not so much that a police officer has capacity to perform any action which an ordinary citizen could do but whether they have the authority to do so. It seems from the decision of the Court of Appeal that police officers have such authority as ordinary citizens, which is not invalidated by their being police officers. The public law limitations, to which the Court of Appeal refers, are where a police officer has capacity but not authority to perform certain acts.
This would fit with the historic nature of police officers in England and Wales – that they are ordinary citizens with no more powers than other ordinary citizens, save where they are specifically granted by common-law or statute. To that end, the Court of Appeal has affirmed that police officers may lawfully do anything an ordinary citizen can do, subject to principles of public law.