If the offer is based on points, then the subjects you take (and consequently gain A,B,C,whatever grade in) are irrelevant.
For example you can achieve 320 points through:
If you miss a grade, you won't have enough points unless you counteract this with a better than expected score in a different module. If I remember correctly, most (if not all) Aston offers are based on points as opposed to individual grades. So, assuming that your offers is in points, nilly is correct and you shouldn't worry about it.