British Isles height (ensemble) - EPSG:9451

WKT

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

WKT2

VERTCRS["BI_height", 
	VDATUM["British_Isles_height_ensemble"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "9451"]]

Data from @esri/proj-codes

{
  "wkid": 9451,
  "latestWkid": 9451,
  "macro": "PE_VCS_BI_HEIGHT",
  "name": "BI_height",
  "wkt": "VERTCS[\"BI_height\",VDATUM[\"British_Isles_height_ensemble\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"BI_height\",VDATUM[\"British_Isles_height_ensemble\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"9451\"]]",
  "description": "British Isles height (ensemble)",
  "authority": "EPSG",
  "version": "9.9(12.9.0)",
  "deprecated": "no",
  "areaname": "Europe - British Isles - UK and Ireland onshore, UKCS",
  "extent": {
    "slat": 49.75,
    "nlat": 61.01,
    "llon": -10.85,
    "rlon": 2.01
  }
}