This looks more like a fern, although that is probably an incorrect description.
No interactions present.
You are right in suspecting it is not a fern; it is a red seaweed. They are not easy to identify without a closer look, especially when detached from their 'moorings' as this one is. It might be Heterosiphonia plumosa which is a red to deep crimson seaweed which appears black when dried. This species has a flattened, fern-like appearance. This site may be helpful. http://www.marlin.ac.uk/phylumdetails.php?phylum=2400
Lat/Lng: 50.7172, -1.8643
OS grid ref: SZ096908