How do I let unis know I'm resitting in M25?

List the subject a second time with the pending date for the retake. You don't need a predicted grade if you're retaking privately or your school won't provide one for some reason.

Hey, thanks a lot! What does retaking privately mean? I will be taking a gap year to revise alone and go back to my school at May to take the exam, therefore I won't be at school to get a new PG. Does that count as privately?

Sorry, might have been clearer if I had said 'studying independantly', IE not with a school or tutor who might be your referee and providing predictions.
As you are entered with your shool as the exam centre, they might be willing to provide predictions. Have you asked them? As above, it's not essential, as most unis will assume some improvement is possible from your first results.

No worries, I asked them and they said since I'm not taking the classes, I won't be able to obtain new PGs. However, I was only 3 marks off the requirement, not sure whether the university will be able to see it though. In this case, although I've technically met the requirements with the 2 required subject and my 4th HL, will I still be automatically rejected or be put at a significant disadvantage without meeting the requirement for the preferred subject?

Unis usually only get your overall grade, not raw marks.
Every uni is different so there isn’t a universal answer to how you’d be considered. But with a pending retake it’s not wildly inconceivable that you could improve so there’s every chance you’ll get conditional offers based on that.