I want to use the OSGB as my co-ordinate system for display in my app, rather than Lat Long WGS84. I've added OSGB using WKID 27700, but can't work out which TFWKID to use. I did a search on the Datum Transformations page (Datum transformations—ArcGIS REST API: Services Directory | ArcGIS for Developers) but the two that refer to Ordnance or Newlyn both yield an error message. I've also tried Googling without success. I'm sure it's simple if you know it, but I don't.