Hi: Two of us will be spending 16 days in England in March/April: 7 days in London and 9 days touring the southeast and southwest. We love historic houses, castles, gardens, museums, and churches. I just saw that there is a Great British Heritage Pass available for 4, 7, 15, or 30 days. Has anyone used it? It seems as though it is both money- and time-saving (since you don't have to wait in ticket lines for purchase or admission). Is that true? Can this be purchased in London, or must we buy it before we leave the US? Thanks very much for your help.