Black. Not sure if the colour is how it started out or because it dried out. Masses of seaweed on beach but nearly all of it soggy.
No interactions present.
The saw tooth edge is a key identifying feature - will be olive green/brown when wet
...is not unusual for the fucoids. Strandline examples will be mostly black.
Lat/Lng: 50.5, -2.5
OS grid ref: SY6777