In considering availability of judicial review
In English law there is but one jurisdiction, namely that of the regular common law courts. In our administrative law, the procedure of judicial review affords courts a supervisory jurisdiction upon the exercise of political power. An individual may apply to the High Court (Administrative Court division) querying the exercise of power by a public body and seek a remedy to the decision made.
Power given via delegated legislation is kept in check via common law remedies. Prerogative orders and decisions made by public bodies that are ultra vires (i.e. exceed the power innate to them) may be set aside. Thus individual rights are to be shielded from the overriding power of the executive.
Any individual who wishes to apply for judicial review must first seek leave from the High Court to do so. Permission to apply may be refused because the application was tardy, in any case the upper limit is three months. The period commences with the initial grievance.  Importantly, this still permits the court to reject applications that though falling within the stipulated three months are still considered to be less than prompt.
Also, Locus standi, or standing, necessitates that the applicant should have sufficient interest in subject of application. 
As the domain of judicial review is public law, the application must concern itself with some aspect of it. An application premised exclusively on private law issues will be rejected.
If the court holds that the applicant has no standing or that there has been delay in applying, then though the case may be heard, any eventual remedy sought could be jeopardised. 
Permission to apply is not automatically refused, but failure to comply with the required conditions means it is less likely that it shall be granted. Notwithstanding this, if the court identifies substantive grounds for judicial review leave may yet be granted. 
The complaint must be directed at a public body; that is, a body which has been statutorily established or which exercises a public function.  In Datafin, the Court of Appeal held that the panel (though established privately) was open to judicial review as its work was fundamental to government-led structure overseeing mergers and takeovers.
On occasion Parliament may choose to oust the jurisdiction of the courts, indeed in legislating to do so the wording of the statutes is often unequivocal e.g. describing decisions as 'final' or 'binding' and without recourse to appeal. Despite this, the judiciary has been unanimous that nothing but the most explicit of formulations will oust their jurisdiction to review the executive and other public bodies. 
The courts diligently uphold time limits on applications for judicial review.  In O'Reilly v Mackman  , a case where public law rights were concerned, the only avenue open for the claimants was judicial review. Use of the civil law procedure was precluded, as initiating a private law claim would have effectively bypassed a procedural safeguard that judicial review surfaces upon public bodies (i.e. standing, time limit and leave to apply).
But conversely, in civil proceedings a defendant is not prevented from presenting public law points as defence; there is no requirement imposed to seek judicial review instead. 
Indeed, when private and public law issues are intertwined e.g. the dismissal of a public sector worker under contract, the courts seem predisposed to allow use of the regular civil procedure by a claimant. The public interest imperative of safeguarding public bodies from tardy and delayed and trivial claims still impresses itself upon the court, however if no such threat is identified then it does not inhibit claimants. 
In the GCHQ case, Lord Diplock divided the grounds upon which a challenge may be mounted (in judicial review) into three categories; illegality, irrationality and procedural impropriety. 
Of these, illegality and irrationality are termed substantive because they relate to the subject of the contested decision itself. Procedural impropriety is identified as the procedural ground as it disputes the correctness of the procedure used to arrive at a decision (rather than the decision itself). The grounds are not mutually exclusive they can and often do appear together.
The ground of illegality serves to ensure that decision makers properly comprehend the legitimate scope of their decision making power and act within its confines.
There are many and varied reasons for a finding of illegality. Amongst these, unlawful sub delegation features prominently, this is where a decision maker ostensibly conveys his power to make a decision to someone who does not carry the legal body to make it.
If statute has empowered an individual or public body to make decisions, they cannot pass this power onto anoqther. The element of discretion involved in decision making distinguishes it from tasks that can be readily transferred. 
Some decisions are quashed because they are based on errors of law or fact. Such decisions may arise from a misunderstanding of facts that were pertinent to determining the extent of a power.  When a power is used for a purpose other than the one Parliament intended when conferring it upon the body or individual then there is illegality.  Disregarding relevant considerations or allowing irrelevant considerations to bear influence on a decision being made also falls into illegality, as it constitutes an abuse of power. 
However, such direction of powers towards improper purposes and the consideration of irrelevant factors is required to materially influence the decision maker if it is to constitute illegality; if it does not, it is falls within the remit of lawful discretion.
If the end determination was guided by good reasons and any irrelevant factors considered had no material influence upon the decision maker, then the decision is held to be valid. 
Other factors constituting illegality include the fettering of discretion. This includes instances where relevant parties are not heard or when individual circumstances are ignored on account of a rigid following policy.  Discretion cannot be bound; the exercise of policies and fulfilling of obligations in an absolute fashion removes the innate flexibility that discretion denotes.
Irrationality is the second limb of the threefold test. Irrationality is an extreme unreasonableness that is “so outrageous in its defiance of logic or of accepted moral standards that no sensible person who had applied his mind to the question could have arrived at it." 
Here where both illegality and procedural impropriety talk to the legal basis of a power, the limb of irrationality is clearly an appraisal of the intrinsic merit of a decision; which demands that there be signs of some logic.
Proportionality requires that means adopted are commensurate with the aim in mind and not excessive. The importance of proportionality within judicial review has increased to a point where it is some consider it to be a more comprehensive alternative to (or at least an addition to) the test of irrationality (Wednesbury unreasonableness). 
Procedural impropriety is where either statutory rules or the rules of natural justice are breached.  There are three core principles of natural justice: fairness, lack of bias and a duty to give reasons.
A decision maker should arrive at decisions fairly. Yet the notion of fairness, however laudable, is often difficult to place and differs from case to case. The maxim nemo judex in causa sua dictates that there should no bias.  The presence of a risk of bias should result in the decision maker being disqualified. 
Where a legitimate expectation is present, an oral hearing may be stipulated as may a requirement to provide reasons for any rejection.  There is also a requirement that any individual know the case against them and be given an opportunity to examine the evidence. Yet there is no general duty to give reasons.  25
Various discretionary remedies can be awarded, separately or together.
Quashing orders nullify the decision made holding it to be totally invalid. Normally, this results when a decision maker acted ultra vires (beyond the powers given to them).The court then orders the decision to be remade, with the court’s judgment in mind.
Prohibiting orders prevents the decision maker from acting beyond their powers, with an effect similar to quashing orders (though prospective in manner). Mandatory orders concern a wrongful failure to act and they enforce compliance with a duty.
Injunctions are orders that require a public body to desist from its illegal actions.
Occasionally, mandatory injunctions that compel action are issued. In emergency scenarios, where there is a likelihood of harm a protective interim injunction may be issued.
Declarations explaining the rights and obligations of each party are sometimes issued. Though not binding, declarations free applicants from compliance to any illehgal decision that the decision maker may subsequently. In this instance, recourse would then be available to other remedies.
Damages are rarely awarded in judicial review cases, applicants proceeding on private law grounds or pursuing a claim on human rights grounds are the sole cases where compensation may be considered.
As all remedies are discretionary, even if the finding of the court is against the public body, it may decide upon offering no award. Reasons for refusing an award include the inequitable behaviour of the applicant by not acting in good faith or by a delay in their application for judicial review. The pursuance of judicial review when more suitable avenues were available is another reason for no award.