I think this is purple laver - found middle shore and attached to pebbles/bedrock here.
No interactions present.
this is much more likely to be umbelicalis but we still cannot be certain.
Most of the Lavers have misplaced common names 'Purple Laver' applies to at least three (according to the WebThing).
I am here because I like to check all posts relating to the one I am intending - now complete
5 Edits later I have come to the correct (but unfortunate) conclusion that we cannot tell, unless there are very strong ID features to be seen or unless the sample has been subjected to microscopic (membrane structure) examination.
I feel like a spoilsport but I am certain of one thing, this is probably not true Porphyra purpurea
Lat/Lng: 51.3877, 1.3671
OS grid ref: TR343707