Phylum: Ochrophyta   Family: Chordariaceae

Chordaria flagelliformis  (O.F. Müll.) C. Agardh, 1817

Slimy Whip Weed

Long whip-like fronds regularly branched and with microscopic filaments giving a slimy feel. Common in the lower intertidal, growing on rocks

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland;
Newton (1931) - A Handbook of the British Seaweeds