Just got back from Starbucks (Cheltenham) and a member of staff gave me this brown plastic wallet. Inside, there is a voucher book with 50p off your drink! (one voucher for each day from 4th Jan to 31st Jan). So in total you save £14 throughout January and it won't cost you anything to ask next time you are instore.
Apparently this is a nationwide thing, so could be worth asking if they have some of these voucher books behind the counter.
My camera isn't working at the moment, so I can't take a picture of the booklet. But, if you go on ebay and type in starbucks £14 voucher, you will see people collecting them from stores and then selling them on!