Brett has a route that people have taken to Brighton twice. I'm sure he won't mind me adding the link to his Bikely site of the route, plus an zip file attachment for a cheatsheet (originally added the the last L2B ride post). Not sure how the roads would be though for middle of the night riding though.
http://www.bikely.com/maps/bike-path/London-to-Brighton
By the way, this is the same day as the Smithfield Nocturne, so perhaps link the two together?? (see post below)
http://www.londonfgss.com/thread5405.html
*EDIT* It seems as though the zip file with the cheatsheet is now corrupted (yes, I know you have to click 'save' rather than 'open'), I'll be able to get it off Brett tomorrow (or he can post it on here himself if he wants).