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Phycodrys rubens is leafy. A. armata is filamentous. It cannot be both. Ursaminor thought it leafy when s/he posted it. Seaweed id is easier if the specimen is photographed spread out on a white background as well as in situ.
Phycodrys rubens is called Sea Oak for good reason - the fronds really resemble Oak leaves.
I suspect this IS Harpoon Weed but we really cannot agree without a little more photo-evidence or a response to Esther H's question. Unfortunately, I think Esther and ursa minor have left the arena. JoC and I are still here, soldiering away!
Lat/Lng: 50.141, -5.0732
OS grid ref: SW805312