This is common in saltmarshes (not on rocks, unlike Spergularia rupicola) all around the sea coast.
All names: Spergularia media (L.) C.Presl; Spergularia marginata Kittel nom.illeg.; Spergularia maritima (All.) Chiov.; Alsine marginata Reichb.
NBN Atlas mapping: Species account : NBN Atlas UK Species Observations database
iNaturalist: Species account : iNaturalist World Species Observations database