In short, you can retake your math GCSE while you're in college, but it wouldn't be part of the university curriculum. You'll need to take separate classes from your undergraduate degree in the afternoon or evening at a local university, or through online classes. You can retake your exams almost anywhere. In fact, most schools and universities will allow you to retake your Math and English GCSE along with your other courses.
You can also take them at a FE university, but you'll probably have to enroll for a year (usually from September to June) and study the course as if it were your first time. Another, more expensive route would be to raise A-levels again through an independent sixth-grade university. On top of all that, there's always the option of online courses if you're working. Although it may not be your first choice, retaking the GCSE or A-level exams could be exactly what you need to get the future you've been dreaming of.
Also, when it comes to retaking A-levels, if you definitely need the grades to access a particular course, or to improve your math and English, it's worth it.