Newlyn - EPSG:5701

WKT

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

WKT2

VERTCRS["Newlyn", 
	VDATUM["Ordnance_Datum_Newlyn"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5701"]]

Data from @esri/proj-codes

{
  "wkid": 5701,
  "latestWkid": 5701,
  "macro": "PE_VCS_NEWLYN",
  "name": "Newlyn",
  "wkt": "VERTCS[\"Newlyn\",VDATUM[\"Ordnance_Datum_Newlyn\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Newlyn\",VDATUM[\"Ordnance_Datum_Newlyn\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5701\"]]",
  "description": "Newlyn",
  "authority": "EPSG",
  "version": "6.7(9.3.0.0)",
  "deprecated": "no",
  "areaname": "UK - Great Britain mainland onshore",
  "extent": {
    "slat": 49.93,
    "nlat": 58.71,
    "llon": -7.06,
    "rlon": 1.8
  }
}