Confidence: I'm as sure as I can be.
Notes: I am suggesting Fucus serratus because, in this photo, the reproductive structures, called receptacles, at the end of the fronds look flttish & lumpy rather than being round and swollen as in Fucus spiralis. Your earlier posting this week (4990833c ) of F. spiralis shows the receptacles in their typical form for this species; they are almost round in outline, and up close, you should see they are surrounded by a narrow rim of sterile frond.
Fucus serratus is a tough, branched seaweed with flattened fronds arising from a disc shaped holdfast . The fronds have serrated margins.
Fucus spiralis is also a tough, branched seaweed, but the frond margins are not serrated.
Desmarestia ligulata is a thin, branched brown seaweed. The branches are narrow at the base and tip. It is an annual so at this time of year it is just beginning to grow. The reproductive organs are microscopic and scattered all over the surface.