I noted that "even my shadow over it didn't phase it; but on touching, it vanished."
No interactions present.
Once the worm has retracted the tube opening would look like an 8 on its side (or infinity symbol) but not quite joined in the middle.
I have never seen them sag like this though it looks as though this one is part in/out of the water (perhaps at the surface of a rock pool)? This could explain its non-reaction to shadow. Fully underwater they form two round splays and are quite skittish and will react to light changes.
Lat/Lng: 51.6835, -5.1779
OS grid ref: SM804030
St Martin's Haven