The exams are taken in May/June and you receive the results in August. If you didn't get the results you had hoped for you can then re-sit your A-levels, taking an exam in some or all of the same subjects, between three and twelve months later, to see if you can improve your results.
If you choose to take more A-levels, then you pick new subjects and start the two-year study again. That's quite unusual because, as far as I know, you can't attend Sixth Form College beyond the age of 18. You can study for A-levels privately at any age, perhaps at evening classes, but you have to pay for the classes and pay to take the exam.
As an example, I took three A-levels at the age of 18 - English, French and Music. I passed them but I didn't get the results I had hoped for, so in October of the same year I re-sat English and French in order to get better results. Obviously, it wasn't the same exam paper!
Student or Learner