This was found in a rock pool right at the water's edge at the turn of the tide. It was not attached to the rock and the 'root' was missing.
No interactions present.
I agree that this is likely to be sugar kelp, although it does look like it has a midrib which would suggest otherwise so I'm not 100% sure. Did you take any other photos of it by any chance?
Also, perhaps you could add a revision and alter the scientific name so that it will appear when other people post observations of the same species - it should be with a capital "S" at the beginning.
Sorry, that's the only photo - but it was taken not far from here, so I'll make another trip and see if I can find some more.
Thanks for your advice,
Lat/Lng: 56.0659, -3.1728
OS grid ref: NT270865
rocky shore leading down to sandy beach.