Found on mud flat-land adjacent to freshwater lagoon along Coastal Path
No interactions present.
My immediate impression was that it is one of the stonecrops , Sedums, but I dont know which. The reddening of the foilage is typical of the stonecrops but the location, mudflats, does not sound like typical stonecrop habitat
Identification added after Paul's steer towards Stonecrop.
Just looked up Mossy Stonecrop and the habitat - damp sandy places , wet in winter -seems to fit the bill.
Crassula tillaea's only South Walian sites (2 of them) are in Pembrokeshire. The invasive Crassula helmsii is more widespread - but either species would be a new tetrad, and possibly a new hectad, record.
The habitat fits Crassula helmsii better.
But I can't identify the plant, even as a Crassula, from the photographs.
Crassula tillaea would have opposite leaves 2mm long which does not fit. It does not look like C. helmsii either. The only suggestion I have is one of the Mesembryanthemums but that is just a guess. Would that it had a flower. Perhaps you could grow it on?
I am out of my depth here but could it be a poorly growing Sedum acre not enjoying its habitat but hanging in there?
In light of feedback
Lat/Lng: 51.3846, -3.3832
OS grid ref: ST038659