Hello from Portsmouth. I've hiked the High Weald Trail, really good I thought, especially the section from Horsemonden to Tenterden, only because I knew this area well as a child. Hiking it as an adult bought back some good memories, especially of Cranbrook.
A tip about the Sussex border path, unless you are a person who must walk every inch of a route, my suggestion is don't bother with Thorney Island, boring, start or finish at Emsworth.
I too Bivi, I like it for stealth camping, in my case when I come out of a pub I really don't want to walk too far in order to sleep.