I used to work in the dispensary - you can claim free prescriptions up until the age of 18, as long as you are also still in full time education (includes uni). You tick the 2nd box down on the prescription sheet for this exemption. If you are aged 19 or over, you cannot claim free prescriptions unless you are eligible for another exemption.
If you do need to pay, then it does sound like getting a pre payment certificate would be better for you (do check though) as it normally works out cheaper.
If you're still confused, you can ask the person who works at the dispensary and they will be able to tell you.