It's different in every school/college. Some automatically give their students one above (so if they got ABBC at AS they'd predict A*AAB) which is utterly ridiculous; most keep it at the same grade - it might come down to begging your teachers to put you up a grade if you really need it for uni!
240/300 is the A grade boundary for AS, 270+ is the 90% UMS boundary so you should get predicted A* but it depends on your teacher. My teachers ignored that and still predicted me an A, proved them wrong though!