There are, said Lord Dyson delivering the leading judgment, two categories of case in which a stay may be justified: (i) where a fair trial could not be held; (ii) where continuing the proceedings would offend the court's sense of justice and propriety [22, quoting R v Maxwell  UKSC 48 at 13, a decision not yet published].
These should not be confused, and it is unhelpful to refer to fairness when discussing the second category (Lord Kerr at ).
The signal here [this is me now, not a Lord] is that what is often referred to as "public policy fairness" should now be called something like "public policy grounds" or "the public policy category".
The circumstances of each case are critical, and the classic cases - R v Looseley  1 WLR 2060; R v Latif  1 WLR 104; R v Horseferry Road Magistrates' Court, Ex p Bennett  1 AC 42 - must be read in this light. Lord Dyson added :
Does the emphasis on the facts of each case erode the value of these cases as precedents? Does the balancing exercise leave the law unacceptably vague? On appeal the question will be whether the decision to refuse a stay was perverse or was one which no reasonable judge could have reached .
Judgments may tend to pull in two directions: to condemn the misconduct of officials while at the same time holding that a stay was not required. That was the position here. Looking at the misconduct from a (considerable) distance – the placing of audio surveillance equipment in a car without lawful authority and giving the impression that permission had been obtained – this was not a case of serious misconduct. But one would think it was, given the stern protestations of Lord Dyson [45-46], Lord Hope [61-62, 68], Lord Rodger [70-71], Lord Brown , and Lord Kerr [81-82]. There were circumstances that mitigated the wrongfulness, summarised by Lord Dyson at [47-50].
The Board found useful the summary of the balancing exercise appropriate to the second category of abuse of process cases (no balancing being appropriate where a trial would not be fair) given by Professor A L-T Choo in Abuse of Process and Judicial Stays of Criminal Proceedings, 2nd ed (2008), at p 132:
The decision in this case may have been finely poised, but it is not surprising: given the extenuating circumstances and the relatively slight (compared to being kidnapped or entrapped) breach of the defendant's rights, and the fact that the offending was importation of cannabis (180 kg, worth over £1m), it is not unreasonable to conclude that the integrity of the justice system did not require a stay of proceedings.
No one likes to disagree with the Privy Council, but I have to doubt whether it is appropriate to address the need for a stay of proceedings as a balancing exercise. Either misconduct is sufficiently egregious to require a stay, or it is not. The balancing factors mentioned in this case are essentially those applicable to the discretion to exclude improperly obtained evidence. The balance is between factors relevant to the seriousness of the impropriety and factors relevant to the seriousness of the alleged offending. This similarity in reasoning raises the question of the relationship between exclusion and the stay of proceedings.
The stay is clearly the more extreme response, and factors justifying it can be put at the end of an imaginary continuum of impropriety, most of which could be a basis for exclusion of evidence unless the offending is too serious. The seriousness of the offence is properly part of the balancing exercise used to determine the admissibility of improperly obtained evidence. But when the level of impropriety is sufficiently high to take the potential remedy beyond exclusion of evidence to a stay of proceedings, is there a level of offending that would make a stay inappropriate? If there were, it would also have to make exclusion of the improperly obtained evidence inappropriate, otherwise where that evidence was central to the prosecution case the proceedings would terminate. Once evidence is ruled admissible after the balancing exercise, because the alleged offending outweighed the impropriety, an application for a stay would be futile. If, on the other hand, the evidence was excluded because the impropriety outweighed the seriousness of the offending, an application for a stay might in extreme cases be successful and if so, that would be because of the seriousness of the impropriety.
There would be some cases where the impropriety, although sufficient to require exclusion of the improperly obtained evidence, was insufficiently serious to warrant the response of a stay. That would be because of the nature of the impropriety, not because of the seriousness of the alleged offending. But where the impropriety was sufficiently serious to justify a stay, it would be sufficient for that purpose no matter how serious the offending. The assessment of the seriousness of the impropriety would be an exercise of judgment, taking into account matters relevant to the impropriety. The judgment would be about whether in those circumstances a stay was necessary to uphold public confidence in the integrity of the judicial process. It would be wrong to say that a prosecution should proceed (because of the seriousness of the alleged offending) even though proceeding would compromise the integrity of the judicial process.
For a glance into the future, see Wilson v R  NZSC 189 (14 December 2015), where the Court described the decision as to whether a stay was warranted in circumstances of investigatory impropriety as a balancing exercise, where the seriousness of the impropriety was balanced against the need to uphold public confidence in the integrity of the judicial process (at , Elias CJ dissenting on this point at ). I side with the Chief Justice on this.
The stay of proceedings can be used in wider contexts than improperly obtained evidence, and in those the seriousness of the alleged offending may well be a relevant factor. Delay, multiplicity of charges, re-litigation of decided issues, improper motive in charging or prosecutorial reneging on an agreement as to charge can all give rise to issues of the appropriateness of a stay, and the seriousness of the alleged offending could properly be a factor in the decision.