Phylum: Chlorophyta   Family: Ulvaceae

Ulva lactuca Linnaeus, 1753

Sea Lettuce

Ubiquitous alga, found in the intertidal and sublittoral zone. The green fronds anchor to substrates with a small holdfast

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