Aaah, your system does seem a bit more confusing than others, though I do think our exam system is a bureaucratic mess itself
Basically our A levels are split over two years. At the end of the 1st year the exams you sit earn you a grade at AS Level (equivalent to half an A level). The A2 exams you do at the end of the 2nd year, giving you the full A level.
The AS level is a qualification in itself though, some uni course can ask for instance, that you have an AS level in another subject as well as your 3 A levels (or 4).