Greenwich is worth the walk
We took the ferry over from the Tower of London to the shore of Greenwich, although the sights on the boat are not as good as the boat ride towards Big Ben, it is a nice 30 min ride to Greenwich. When the old Navy ship, which was once the fastest ship in the world, is in sight, you are near your site. The Royal Navy grounds are beautiful and the park is a great day to relax and enjoy the day, weather permitting of course. The 15 min walk will take you to the to the reason we went there, stand in both the Western and Eastern hemisphere. The walk up the hill will be hard for small children and the elderly. Many people had to take breaks on the way up. It is well worth the walk when you reach the top. The view is great and the pictures you can take of the line and the GMT clocks at the top are pretty neat. You had to get tickets which are free at the top to get in, which is strange, but I guess it is there way of regulating the amount of people at the top. You might have to fight others for a good picture, when it is crowded. Well worth the trip and if you mix it with the Tower of London tour, like we did, there will be plenty of time to enjoy everything. The boat runs all day back and forth and leaves from the Tower of London Peir. Cost is around 6 or 8 pounds if I remember right, round trip.