No interactions present.
... is Spartina anglica which is a rather recent allotetraploid derivative of Spartina maritima and the introduced Spartina alterniflora. But all three species are present in Essex, so you can't use distribution as a short cut.
Lat/Lng: 51.7754, 0.9046
OS grid ref: TM004125
growing alongside shingle path across saltmarsh. Covered by the higher tides. There is a lot of it, on average about knee high.