Ask at any retailers for a student discount, many will ask to see ID, uni' ID will work.
The National Union of Students (NUS) has negotiated discounts with national companies and they expect to see a NUS card.
It is indeed Â£10 for a NUS Extra card. Worth every penny as one discount can be in excess of a Â£10. I've had one since 2001 and wouldn't be without one. One day I'll no longer be entitled unless I do a course that attracts student status.
Get one. Save a packet on nights out - downside is you have to go the bar for your mates who do not have a yellow card (different to NUS card, local bars recognise these, costs Â£2 but gets pints for Â£1 for the whole year).