A bit more than an inch across
No interactions present.
....am on the look out for this phaeophycea myself in east Kent as some has been seen here and not recorded here. Even in your area it is not common. Non-native, believed to be introduced from the Pacific.
Colpomenia peregrina seem to come onto the shore here along with big deposits of seaweed - I think especially with red seaweeds but haven't made a study to check.
Blog - Loose and Leafy - http://looseandleafy.blogspot.com/
Plenty here and LOADS of Leathesia difformis to come later.
"It was introduced to France with juvenile Crassostrea virginica from the Pacific coast of North America."
Lat/Lng: 50.5, -2.5
OS grid ref: SY6777