There are 3 conditions that allow you to get NHS prescriptions free of charge. Epilepsy, Diabetes and something else which I can't remember.
I've been diagnosed as a borderline diabetic which means I have diabetes but as yet don't require medication for it. I can therefore have free eye tests but not free medicine. However, the diabetes has been caused by other long term conditions that I have and is being controlled by increasing my medication for these. So I now have to take 7 different medications every day ( to be increased next month to include HRT medication - the joys of being an old woman) to ensure my diabetes doesn't increase to a level requiring medication which would then result in no prescription charges at all.
I think I'll have to get another job to pay for it all. Even with the NHS "loyalty" card, where you pay up front and save money it's a fortune for an impoverished student :)