The rose is a Christian symbol, symbolizing charity and associated with the Virgin Mary. During the mediaeval War of the Roses, when the families of the royal houses of York and Lancaster were fighting for the throne, the House of York used the traditional white rose (representing purity and good) as their symbol, while the House of Lancaster used a red rose (representing the blood of Christ). The Lancaster side won, so the red rose became the symbol of England. Why it's the symbol of the Labour Party, however, is a different story :-D