I downloaded the Shopmium App and you can receive cashback for food and drink that you buy and sometimes you receive 100% of the money back so the item you've bought is free. If I refer users to sign up with my referral code: KMYGYUEM then you can claim a free 100g bar of Green & Blacks chocolate.
Note this only works if you download the app onto your phone and register on your phone. If you register on the website the claim button for the chocolate won't appear.
To claim your chocolate, you buy the chocolate bar from the shop where the map shows you, in my case my nearest shop was Asda. You scan the barcode of the chocolate bar and upload a pic of the receipt and then the money you paid is transferred via your method of payment in a couple of days.