I read all three cases in footnote 193, but they don't explain the boldening.
At an earlier time, the courts rejected the use of the private law doctrine of estoppel in easing the plight of someone who had relied on an assurance from an official only to find that it was not binding because it was thought to allow officials to play fast and loose with legal rules and the limits of a public authority’s powers, duties or jurisdiction.193 Thus estoppel could not affect the obligation to perform a statutory duty. The House of Lords reconsidered the issue in 2002, in which Lord Hoffmann said:
It seems to me that in this area, public law has already absorbed what is useful from the moral values which underlie the private law concept of estoppel and the time has come for [public law] to stand upon its own two feet.194
As a result, the analogy between legitimate expectations and estoppel is not one which is likely to be of any utility in the near future.
193Maritime Electric Co v General Dairies Ltd  AC 610; Rhyl UDC v Rhyl Amusements Ltd  1 All ER 257; Essex Congregational Union v Essex CC  AC 808.
Bradley, Ewing. Constitutional and Administrative Law (2018 17 ed). p 667.