The DPP guidelines on prosecuting offences involving the media issued 2012 can be found HERE 

To prosecute, the CPS must be satisfied that 2 tests are met:
a)     that there is sufficient evidence to provide a realistic prospect of conviction. This means that an objective, impartial and reasonable jury (or bench of magistrates or judge sitting alone), properly  directed and acting in accordance with the law, is more likely than not to convict.
b)     prosecutors must go on to consider whether a prosecution is required in the public interest.
The question of public interest is complex, involving the weighing of a number of factors, essentially relating to the gravity of the conduct publicised on the one hand, and on the other the degree of harm to the victims of the offence.

“Journalistic privilege”:
Prosecutors are reminded that under section 10 of the Contempt of Court Act
1981, no court may require a person to disclose, nor is any person guilty of
contempt of court for refusing to disclose, the source of information contained
in a publication for which s/he is responsible, unless it be established to the
satisfaction of the court that disclosure is necessary in the interests of justice
or national security or for the prevention of disorder or crime

The guidelines can be criticised for concentrating on the rights and duties of journalists, and ignoring the problem of policing the police –  but this may be a matter of available criminal law, since the only 2 offences realistically applicable to police officers are:
(1)     under the Official Secrets Acts (where there has to be an additional element to the simple malicious or corrupt release of confidential information – in broad terms there has to be damage to international relations, or causing an offence to be committed )
(2)     under the serious charge of Misconduct in Public Office (carrying a maximum sentence of life imprisonment)

The eternal question remains – quis custodiet ipsos custodes?