In a space of a few hundred metre i saw 4 Polish Grocery Stores that were selling canned food, cheeses, soups etc but what is really good are the cured meats and sausages which are not too common in English butchers. Try the Keilbasa sausage, Kiszka, Bockwurst, or Knockwurst.
If you are in search of reading material check the books out at the charity shops of the The British Heart Foundation or the Cancer Research Shop. You should find some books in good condition hidden away among the shelves and your money goes towards helping others.
What to pay: £2-3
The Treaty Centre is the shopping mall with such stores as Debenhams, various boutiques, shoe shops and mobile phone shops as well as the library. Also a good cafe by the entrance at the eastern end.