From what I can remember, you don't qualify for free dental care just by virtue of being a full time uni student (only students under 19 receive free care), but you can claim for free treatment on the basis of having a low income. You will need to complete a HC1 form, which you can probably obtain from your dentist or local social security office.
Look on ppa.org.uk and type in 'low income scheme' in the search box. You'll see lots of information that explains the position of students and healthcare costs. The upshot is, you need to complete the HC1 form.