Don't limit yourself to the standard drinks. There are plenty of options that are well under £4, such as iced coffee. If you think that spending less means boring drinks, think again. Check out some the most creative Starbucks mystery menu concoctions under £4.
And if you must go the latte route, try the caffe misto, which is made of of half coffee and half milk. It is not only cheaper, but also has less calories!