Well A few things that helped me when I was prety much broke in Edinburgh...
Grocery stores ( dispite the line ups), Milk ( cheaper than water in some places), and not drinkin all that much.
If you even touch a pub for food or drink your looking at like 3 meals you could buy at a grocery store...Bread, Deli meats, canned chicken and tuna did me well, if your staying somewhere that has a kitchen, even better. you get to cook some stuff.
I don't know how, but when walking about, I was finding places that were selling milk for 40P and water for 85P...That blew me away...so I took a keen liking to milk for the whole time I was in Edinburgh.
Where are you staying while you are there???