It’s not illegal as long as the process is fair, reasonable and non discriminatory, watch out for kids, carers and religious commitments, Normal practice would be minimum notice period equal to length of forced holiday to be taken, and deal with any issues that present.
You also don’t have the legal right to take holiday when you want, although if a boss starts to reject requests too often people will just call in sick, especially if youve got day one full sick pay. Managers could just keep rejecting holiday so that it gets lost at year end if thats what your contract says, but that would be a no brainer road to nowhere if it went to ET.
Obviously the lower down the ethics tree your employer sits the more likely they are to dump on you.
I don’t really do ER very much these days, limited to SAs and complicated stuff, but I don’t think things would have changed too much since I last updated a Holiday policy.