this isn't very helpful but just something I was thinking recently:
the thing about walking the Ridgeway is that (and correct me if I'm wrong) there is no water up there? If it's chalk downland then all the streams will trickle out at the bottom.
So that's a pain for the modern walker (that's my link to your question) - but what about the prehistoric walker? Surely it was a pain for them too, and whatever animals they were with? Why isn't the track at the bottom of the ridge, not the top? Fair enough, you can see better from the top.
Can anyone enlighten me?
By the way, where will you be starting from?