If you have failed your GCSEs, A Levels or didn't get the grades you want, you may want to retake, so you can continue with the A Levels or degree you want. If you failed AS exam(s), you can retake with your A2 exams, else you may need to spend an extra year on retakes.Before you decide to retake you be sure
1. You are confident of getting a good grade. In my experience a typical student will improve by about two grades as a result of a retake.
2. A pass is needed to continue with the course you want.
I would advise a retake if you are in any doubt whatever to retake or not. The longer you delay, the more you will have to pay. Missing several entry deadlines can make the cost of retaking an exam eye watering. You can feel sorry for one year for having to retake and not for the rest of your life for having failed at life.
Retaking a University exam is possible in the first two years and is done in the summer of the year of your first attempt. Your mark is usually capped at a pass. First year exams typically contribute 0 - 10% towards your degree, so a first year retake will probably not affect the class of your degree.