Hi,
Well FWIW I live in Brighton and have talked to cyclists that have done the London to Brighton.
They all say the same thing, Ditchling Beacon is an utter killer 50 miles into
If you think it is hard on the l2b try the Puncheur Sportive. Starts in Ditchling heads up into the Ashdown with several short but punchy climbs and loops back down and finishes at the top of the beacon. 65 miles 4200 feet. The L2B is a diddle in comparison.