I'm assuming that you might not be considering early entry, so if your school does exams or have some kind of measure before submitting final UCAS grades, you have time to raise it. Work over the summer holidays if possible on upping your grades and ask for a re-evaluation from some teachers and hopefully they'll be able to predict you higher once they see that you've improved.
Other options include switching to Core Maths, re-doing the year/ resitting, taking a gap year and ensuring that your actual A level grades are higher to apply for the next year or possibly a Foundation Year with universities that provide it for your course.
AS levels are good, but ultimately these days universities understand that it's not as important anymore as less and less people are doing them. If you focus on your A level exams in Year 13 and putting all the effort in, you won't necessarily have to resit the year