The public body will use that power
The flaw in the regulation can only be discovered once the power has been used and someone aggrieved by that takes the matter to court. The regulation has to be found unlawful by the court.
At present, the court has discretion in determining what happens. They may decide it was void ab initio and that all acts under its colour never happened. However, if that will cause administrative chaos or affect the rights of third parties, they may decide that the egg can’t be unscrambled and that the regulation is instead voidable:
…suggests that values can influence the classification of a bye-law as void or voidable (or, to eschew the language of conceptualism, whether an invalid bye-law can nonetheless have continuing effects): here, good administration (a concern to avoid chaos) and democracy (a concern to give effect to provisions passed by Parliament) are seen to play a role. Lord Browne-Wilkinson certainly argued that good administration could be relevant: “I am far from satisfied that an ultra vires act is incapable of having any legal consequence during the period between the doing of that act and the recognition of its invalidity by the court. During that period people will have regulated their lives on the basis that the act is valid. The subsequent recognition of its invalidity cannot rewrite history as to all the other matters done in the meantime in reliance on its validity”. As Lord Slynn of Hadley put it, “[t]he unscrambling may produce more serious difficulties than the invalidity”.
Boddington v. British Transport Police  UKHL 13;  2 AC 143;  2 All ER 203;  2 WLR 639 (2nd April, 1998)