MRH21-IRF - EPSG:9866

WKT

GEOGCS["MRH21-IRF", 
	DATUM["MRH21_Intermediate_Reference_Frame", 
		SPHEROID["GRS_1980", 6378137.0, 298.257222101]], 
	PRIMEM["Greenwich", 0.0], 
	UNIT["Degree", 0.0174532925199433]]

WKT2

GEOGCRS["MRH21-IRF", 
	DATUM["MRH21_Intermediate_Reference_Frame", 
		ELLIPSOID["GRS_1980", 6378137.0, 298.257222101]], 
	PRIMEM["Greenwich", 0.0], 
	CS[ellipsoidal, 2], 
	AXIS["Latitude (lat)", north, 
		ORDER[1]], 
	AXIS["Longitude (lon)", east, 
		ORDER[2]], 
	ANGLEUNIT["Degree", 0.0174532925199433], 
	ID["EPSG", "9866"]]

Data from @esri/proj-codes

{
  "wkid": 9866,
  "latestWkid": 9866,
  "macro": "PE_GCS_MRH21_IRF",
  "name": "MRH21-IRF",
  "wkt": "GEOGCS[\"MRH21-IRF\",DATUM[\"MRH21_Intermediate_Reference_Frame\",SPHEROID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]]",
  "wkt2": "GEOGCRS[\"MRH21-IRF\",DATUM[\"MRH21_Intermediate_Reference_Frame\",ELLIPSOID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],CS[ellipsoidal,2],AXIS[\"Latitude (lat)\",north,ORDER[1]],AXIS[\"Longitude (lon)\",east,ORDER[2]],ANGLEUNIT[\"Degree\",0.0174532925199433],ID[\"EPSG\",\"9866\"]]",
  "description": "MRH21-IRF",
  "authority": "EPSG",
  "version": "10.039(13.1.0)",
  "deprecated": "no",
  "areaname": "UK - Cardiff to Lincoln",
  "extent": {
    "slat": 51.35,
    "nlat": 53.26,
    "llon": -3.27,
    "rlon": -0.36
  }
}