There is a right to be told of how decisions about whether or not to prosecute are made: R (Purdy) v Director of Public Prosecutions  UKHL 45 (30 July 2009).
Article 8 of the ECHR provides:
"1. Everyone has the right to respect for his private and family life, his home and his correspondence.
"2. There shall be no interference by a public authority with the exercise of this right except such as is in accordance with the law and is necessary in a democratic society in the interests of national security, public safety or the economic well-being of the country, for the prevention of disorder or crime, for the protection of health or morals, or for the protection of the rights and freedoms of others."
The Appellate Committee ordered the Director to publish a specific policy on decisions to prosecute allegations of assistance in suicide. Here there was a possibility that assistance might be given to a person who wanted to commit suicide lawfully in Switzerland, by helping them leave the United Kingdom for that purpose. The potential assister wanted to know about the likelihood of being prosecuted.
The common law on liability for assisting what would be an offence in the jurisdiction where the assistance is given, but not an offence where the full offence is committed is reasonably clear, although Lord Phillips did not think it was necessarily as settled as did Lord Hope who delivered the leading opinion. Whether that jurisdictional issue was settled or not, the real issue in this case was access to information about decisions to prosecute.
Baroness Hale noted (57 – 58) the recent Parliamentary debates on the issue of assisted suicide, which did not result in legislation. There were drafting difficulties, and indications of a preference to leave the matter to prosecutorial discretion. Since a major objective of the criminal law is to warn people of when they might be punished (59), article 8(2) was engaged (62). Lord Brown said article 8(2) required accessability and foreseeability in assessing how prosecutorial decisions are likely to be exercised (85). Lord Neuberger said the applicant was entitled to guidance on that (106).
Lord Hope pointed to the crucial circumstances that a Code guiding prosecutorial decisions should address (53):
"...There could be [... cases] unsuitable for prosecution where, for example, it could be said that those who offered assistance stood to gain an advantage, financial or otherwise, by the death. An assistant who was not a relative or a family friend might have to be paid, for example, and a relative might derive some benefit under the deceased's will or on intestacy. The issue whether the acts of assistance were undertaken for an improper motive will, of course, be highly relevant. But the mere fact that some benefit might accrue is unlikely, on its own, to be significant."
"54. The Code will normally provide sufficient guidance to Crown Prosecutors and to the public as to how decisions should or are likely to be taken whether or not, in a given case, it will be in the public interest to prosecute. This is a valuable safeguard for the vulnerable, as it enables the prosecutor to take into account the whole background of the case. In most cases its application will ensure predictability and consistency of decision-taking, and people will know where they stand. But that cannot be said of cases where the offence in contemplation is aiding or abetting the suicide of a person who is terminally ill or severely and incurably disabled, who wishes to be helped to travel to a country where assisted suicide is lawful and who, having the capacity to take such a decision, does so freely and with a full understanding of the consequences. There is already an obvious gulf between what section 2(1) [of the Suicide Act 1961[UK]] says ["A person who aids, abets, counsels or procures the suicide of another, or an attempt by another to commit suicide, shall be liable on conviction on indictment to imprisonment for a term not exceeding fourteen years"] and the way that the subsection is being applied in practice in compassionate cases of that kind.
"55. The cases that have been referred to the Director are few, but they will undoubtedly grow in number. Decisions in this area of the law are, of course, highly sensitive to the facts of each case. They are also likely to be controversial. But I would not regard these as reasons for excusing the Director from the obligation to clarify what his position is as to the factors that he regards as relevant for and against prosecution in this very special and carefully defined class of case. How he goes about this task must be a matter for him, as also must be the ultimate decision as to whether or not to prosecute. But, as the definition which I have given may show, it ought to be possible to confine the class that requires special treatment to a very narrow band of cases with the result that the Code will continue to apply to all those cases that fall outside it.
"56. I would therefore allow the appeal and require the Director to promulgate an offence-specific policy identifying the facts and circumstances which he will take into account in deciding, in a case such as that which Ms Purdy's case exemplifies, whether or not to consent to a prosecution under section 2(1) of the 1961 Act."
This is an interesting example of the common law going where Parliament fears to tread.