Audax rides

my advice is to reduce elevation you want to go through edenbridge, which is the flattest area. I don't know too much about the north downs so unsure where to get out of london, but Layhams will probably have the easisest gradient ? then you want to go east of Ashdown forest (shame as it has the best vistas) and Crowborough which are on lumps, end enter eastbourne from the marshes east of Hailsham. You should be able to keep it under 100km each way, and maybe 1150/1200 elevation, depending on the start location.

Just another to throw into the mix, one way to Hastings but easy to chop the end and add a leg back, not too much elevation