"London streets are paved with gold" is a saying that came from the 19th century story of Dick Whittington and his cat, loosely based on the 14th century Lord Mayor of London, Richard Whittington. The saying, which expresses the idea of a "land of opportunity", is partly ironic, since Dick Whittington found when he went to London that the streets were in fact grimy and poverty stricken. The second irony was that with persistence, belief, and luck, Whittington was able to become successful.
The origin of the phrase may be Revelation 12:21, where the New Jerusalem is described: "The twelve gates were twelve pearls, and each gate was made of a single pearl. The street of the city was made of pure gold, as clear as glass." (International Standard Version)