When studying in London and going to restaurants and shops, you will see that many of them offer student discount, but some of them only accept your National Union of Students card. In order to get the card you must first register on their website and order the card. The card options are a one year card, two year, or three year card. The prices are £12, £22, £32 respectively.
The places that offer discounts are Amazon, Pizza Express, Odeon and there are plenty of others.
It is definitely worth it to buy the card because student life is not cheap and even if it's just a small discount it still helps a lot in the end.
There's more information about the card and its discounts on the website: http://www.nus.org.uk/en/nus-extra/