Flannan Isles - EPSG:5748

WKT

VERTCS["Flannan_Isles", 
	VDATUM["Flannan_Isles"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["Flannan_Isles", 
	VDATUM["Flannan_Isles"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5748"]]

Data from @esri/proj-codes

{
  "wkid": 5748,
  "latestWkid": 5748,
  "macro": "PE_VCS_FLANNAN_ISLES",
  "name": "Flannan_Isles",
  "wkt": "VERTCS[\"Flannan_Isles\",VDATUM[\"Flannan_Isles\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Flannan_Isles\",VDATUM[\"Flannan_Isles\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5748\"]]",
  "description": "Flannan Isles",
  "authority": "EPSG",
  "version": "6.15(9.3.0.0)",
  "deprecated": "no",
  "areaname": "UK - Flannan Isles onshore",
  "extent": {
    "slat": 58.21,
    "nlat": 58.35,
    "llon": -7.75,
    "rlon": -7.46
  }
}