Leaves are only about 4mm across, but have distinctive dark blotches. Growing on a shingle beach near the high water mark.
No interactions present.
Looking at it again, I'm less sure about thrift, still don't think it is Littorella, esp. as it is on the sea coast. I've put up two more possibilities to see if they get any votes.
Don't know what it is, but you are right that it is not Littorella uniflora.
Lat/Lng: 57.26, -5.9432
OS grid ref: NG623257