Psychle and I did this a few weeks ago; flat, easy route along the River from central London, then onto the Wey Navigation canal to Guildford, then along the Downs Link to just north of Shoreham then you can take the coastal path into Brighton. Roughly 90 miles that. Of course you have to add in another 10 miles somehow to get the fabled Wunundred. 🙂
Oh, and you have to do it on a singlespeed. 8) Actually it’s flat, so SS is no problem.
Not on roads for most of it, but easy tracks. Much nicer anyway, away from traffic and people.
http://www.singletrackworld.com/forum/topic/london-to-brighton-offroad-its-a-looong-way-for-some-fish-chips
Dunno about a GPS route though, I think Psychle’s packed up halfway through. You don’t need one though tbh.