Phylum: Rhodophyta   Family: Delesseriaceae

Delesseria sanguinea (Hudson) J.V. Lamouroux, 1813

Sea Beech

As the common name suggests, the fronds look like beach leaves, but can be quite ragged when found washed up with the tide. A common species, found subtidally, where it's a characteristic member of the understorey flora in kelp forests

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland;
Hiscock (1986) - A Field Key to the British Red Seaweeds