I'm pleased to see the Arun Valley line mentioned numerous times in this thread. When I've been in the south making use of a Southern Daysave ticket I always make sure that I use this route a least once when travelling to and from the south coast from London. A great scenic ride with lovely river views and great countryside. I did this route from Ford to Horsham a few weeks ago and changed at Horsham for a train to Victoria via Dorking and Sutton. A much more pleasant route from the south coast to London than via the Brighton main line.