Dill I highly recommend calf guards, I suffer from tight calves when I run and hence slow recovery from long runs, they've almost completely eliminated the tightness, I wouldn't do anything over 30 miles without them now.
I'm thinking brooks cascadia but I may bring a pair or road shoes for the Thames path legs, haven't run those but most of it is fairly hard packed isn't it?
Re the base layer, is there a need to carry a spare or did it mean wear it? Similarly with the legs, I took it to mean if you wore tights then there was no need for another layer