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.
THREE MARINE PROJECTS
Flat periwinkles are never ribbed. This is, without any doubt, Littorina littorea. The second apertural image shows clearly the diagnostic pure white columella with a dark basal mark. See https://flic.kr/p/pfuSfj both it and your image have the juvenile ribbing and thin outer lip of L. littorea.
There is an account of how to differentiate Littorina obtusata from L. fabalis at https://flic.kr/s/aHskPGCpTE
Lat/Lng: 51.3834, 1.3219
OS grid ref: TR312701
higher eulittoral zone amongst fucoids.