Even if you didn't say you were resitting your maths, if they give you an offer with a certain maths grade in it, they will expect you to be able to produce proof that you have achieved that grade. Many students have to resit maths, and it's expected that you make the effort to do so. I needed a grade B in GCSE maths to get onto my uni course, and I was asked to produce my certificate for that and my A Levels (even though they'd already got those results) on the first day when I arrived.