The Legal and Institutional Background
The doctrine of legitimate expectation operates as a control over the exercise of discretionary powers conferred upon a public authority. The typical reason why discretionary powers are conferred upon a public authority is to ensure that they are exercised having due regard to the particular circumstances of individual cases coming before the decision-maker – ie in circumstances where Parliament was not confident at the time of passing legislation in predicting all circumstances and how individual cases should be resolved. It is often difficult to tell in advance of concrete situations arising precisely how an authority should act; and that may be as true for the authority as for Parliament itself. This reasoning is inherent in the rule forbidding a public authority which has a discretion and adopts a policy as to its exercise from following that policy without having due regard to the specific facts of the particular case: British Oxygen  AC 610.
The doctrine of legitimate expectation is the converse of this situation: it operates to say that, subject to certain conditions, a public authority which adopts a policy should be required to follow and apply that policy in cases subject to it, without being permitted to depart from it. It applies in cases where the decision-maker has committed itself in advance to a particular course of conduct in a particular class of case (defined in more or less general terms), without reference to the specific facts of individual cases. The key issue, therefore, in legal terms, is to explain by reference to normative considerations when and why (or why and, therefore, when) the policy maker will be required to follow its own pre-determination of the outcome of the case without being free just to change its mind when the specific facts of an individual case are before it.
In addressing that issue, it is important to understand that the problem which presents itself is one of public law, and that private law analogies are of limited assistance: see R (Reprotech (Pebsham) Ltd v E Sussex CC  1 WLR 348, HL, at  per Lord Hoffmann (indicating an important shift from the cases in the infancy of the development of the doctrine, where the private law analogy was used as the normative foundation for the doctrine: see esp. Re Preston  AC 835, 865-7 per Lord Templeman). Viewed from the perspective of public law, the protection of legitimate expectations is both greater but also less than the protection of comparable interests under private law (particularly in contract or estoppel cases):
The protection is greater, in that in some cases no absolute promise or consideration or detrimental reliance may be required. It may be sufficient to say that the adoption of a policy by the decision-maker informs and strengthens the application of general requirements of lawful behaviour on the part of public authorities, including in particular (in the case of representations as to the procedure to be adopted) the requirement that a public decision-maker act fairly and (in the case of representations as to the substantive outcome in a class of case) the requirement that a public decision-maker act consistently and rationally (cf Matadeen v Pointu  AC 98. In each sort of case, it seems that the specific adoption of a policy by the decision-maker will strengthen rights which the individual enjoys under general public law. But then, it is important to identify to what extent the adoption by the public authority of a policy in advance adds to the rights which the individual already enjoys under general public law;
The protection is less, in that the interest of the individual in having the policy or representation complied with in their case may be overridden by countervailing public interest considerations. Short of a binding private law contract or estoppel, the hands of the public authority...