Phylum: Ochrophyta   Family: Phyllariaceae

Saccorhiza polyschides (Lightfoot) Batters, 1902

Furbelows

This is the largest European seaweed. It's an annual and hence very quick growing, attaining up to 5 metres in length in a matter of months. The stipe is broad and flat with frilly edges which dissipates wave energy so that it can survive in the surf. The holdfast is large and covered in warts when mature (charmingly called a sea hedgehog). An opportunist that grows from extreme low water and beyond

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland