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https://ridewithgps.com/routes/39632158
This is my best shot. Go on your way to Brighton but then swing east before Turners Hill and then try and make the most of the space between the Weald and the South Downs. I can't speak for the roads down there, look a terrible tangle, but there might be some usable ideas in there. Only issue is that it's a fair bit longer than 200k, more like 250k. With some A/B road compromises in that route you could probably get it down to 225km. For the inexperienced I'd say too many hills is worse than too far.
I'm trying to route the flattest 200k I can from SE London down to Eastbourne and back, so thought I'd check here for inspiration. The only chat I could find was @hippy trying to build the lumpiest version possible, whereas I need to do the opposite. I'll be riding with a group of mixed ability and worry anything much more than 2000m total climbing could end up being a loooooong day and not much fun for some. Is it doable?