Blue Willow is a pattern that has been in constant production for a long time. The image tells the tale of Chinese sweethearts and has been adapted in various forms by many companies.
Charles Allerton and Sons produced transferware in England from 1859 - 1942. Before you try to learn the value of your pieces, you should find out when it was produced. Age and condition mean a lot. I have seen Blue Willow by Allerton for sale for a wide variety of prices. Older, rarer pieces will be more valuable.
A maker's mark indicated who produced the dishware. Most manufacturers slightly changed their marks over time. You can find a chart of Allerton marks and the changes they made at an online site called The Potteries.org. There you can get a general idea of the age of your pieces. Then you can easier learn the value.
Once you know the age of your blue Willow, you can scan online auction sites for value. Ignore the highest option. You can also check out price guides on Kovels, Replacements, or Worthpoint.