In the higher eulittoral zone. All the Periwinkles here tend to be black rather than yellow, red, green, banded, etc.
No interactions present.
Graham, isn't there a flat P with this pronounced ribbing somewhere?
It also looks very thin shelled.
Lat/Lng: 51.3834, 1.3219
OS grid ref: TR312701
higher eulittoral zone amongst fucoids.