I hate hats but did buy one on the front at Sidmouth only to find my head got hotter. So its the walking trousers, as all year round, zipped off to shorts, vest type tee shirts but never the armpit display type, not uncommonly SOCKS with my all terrain sandals, to cushion the tootsies after a proper walk in boots.
Carry a car door and if it gets too hot you can wind the window down.