I've plumped for "marine" habitat as it was exposed at (average) low tide rather than "coastal".
No interactions present.
It looks like a closed up sea anemone. Was it like hard jelly? Once the water has left they shut down. When submerged they open and look like beautiful frilly flowers.
I didn't get close enough to touch it. It was in an awkward place and having forgotten to eat breakfast or take a lunch with me I was feeling a bit hungry and too wobbly on my feet to clamber down to it. Also I had gone specifically to photograph something else (Sabellaria alveolata) and was distracted by the profusion of choice of that subject.
Thanks for the suggestion and to Dorset.Mushroom.Hunters for the ID.
Lat/Lng: 51.50304, -3.75029
OS grid ref: SS786796