For what “apparently formalistic procedural reasons” would courts deny potential illegality to continue unchallenged?

Question

I read the judgment on BAILII, but still don't grasp the meaning of the boldening.

Bradley, Ewing. Constitutional and Administrative Law (2018 17 ed). p 683.

      Claims for judicial review must be made promptly and ‘in any event not later than three months after the grounds to make the claim first arose’, but the period may be shorter for a particular claim if legislation so provides. If the court considers that the case is one which requires urgent action (for instance, a challenge to school admission decisions or where third parties are affected), it may refuse permission for a claim that is not made promptly, even within the three-month period. The court may extend time if there is a good reason to do so, but the parties may not extend time by agreement. In practice, where there is a ‘clear-cut’ case on the merits, the courts are not minded to allow the potential illegality to continue unchallenged for such apparently formalistic procedural reasons.¹⁷

¹⁷ R (Finn-Kelcey) v Milton Keynes Borough Council [2008] EWCA Civ 1067, [2009] Env LR 17; C J S Knight [2009] JR 113.

Answer 1

The “apparently formalistic procedural reasons” is the arbitrary requirement that applications be lodged “promptly”

If the conduct the subject of the review is clearly illegal and no third parties would be adversely affected by bring it to an end, the court is likely to waive the “promptly” requirement.