This link says nothing about having to be on medication and gives you the other categories that can get free eye tests.
http://www.nhs.uk/chq/Pages/895.aspx?CategoryID=68&SubCategoryID=157
Also if you work with vdus your employer is responsible for you having an eye test for that and if you need glasses specifically for a vdu then they should give you a set amount towards the cost of those glasses.
I thought that you always got free eye test if you are diabetic and a discount on the lenses (or free if your on low income) I have always paid for the frames though! 🙄