From nature's table: Seaweed, treasure on the seashore

Seaweed. Tell me about seaweed.

It appears they are marine macro algae, plant-like organisms that usually live attached to rock or other hard places in coastal areas. (Between a rock and a hard place

But its beauty is just the start. Amid the flotsam and jetsam on our beaches lies a rich abundance of brown, green and red seaweeds that can be put to good culinary and cosmetic use. 

Poking around a bit I found this great guide: Ten to look out for are here https://www.countrylife.co.uk/nature/a-simple-guide-to-british-seaweed-how-to-spot-it-how-to-cook-it-159632

HOWEVER 

If you are thinking about cooking with seaweed beware says Fergus the Forager on his very useful website: 

Seaweeds are known to take up heavy metals, radionucleotides and various other pollutants. Therefore do not harvest from areas close to places like Sellafield and or areas of heavy industry. 

http://fergustheforager.co.uk/seaweed/