I would also suggest Bruges, but via Folkstone. From the east end its a good run along the coast on the first day perhaps splitting this into two 50 mile days (split it somewhere around maidstone/the weald) then ferry to Calais, its around 70 miles to Bruges dead pan flat and very open. So perhaps do that in one day and return to folky on the last day for a fast train home (or a ride if you fancy it).
Alternatively, I'd head to the new forest. Its 100 miles there so perhaps a 4 day ride there and back for you, with a fantastic days riding around the forest itself in the middle. 5 days all in