Samba is a mobile data sim provider which gives you free data in return for watching advertising. Typically about 3Mb per video. They often have double data adds and weekends where all ads are double data and this is usually when I stock up on data for my iPad sim (I had to cut the sim down to nano size though).
Sim cards are £5 or £2.50 if someone sends you a referral email. Its great for a not often used device such as a tablet that's usually on wifi.