Don't forget that English "willow pattern" - it doesn't exist anywhere else, and was concocted by Josiah Spode - is not porcelain but tin-glazed earthenware, produced, like Dutch Delft, in imitation of early Chinese blue-and-white porcelain. At the time of its introduction Europe did not have the secret of porcelain production, and the real thing imported from China was terrifically expensive. Virtually all the examples of Chinese painted porcelain exported to Europe at that time were blue and white, for the reason touched on by Shaneystar, so naturally so were the European "lookalike" products. Imitation Oriental ceramics became so popular that many manufacturers introduced ranges in the same tin-glazed earthenware but depicting typically English scenes - usually of rural market towns, hunting scenes or idyllic village greens. Incidentally, I've never seen any willow pattern with a metallic edge.