EDIT - I now know that it is because uploaded CSV files with eastings and northings on the British National Grid datum still need to be geocoded, I guess since the main ArcGIS platform works in Lat/Lon. ArcGIS Online geocodes based on a free British National Grid geocoding service provided by ESRI UK.
My issue now is how would I replicate such a service behind a firewall given that my ArcGIS Enterprise system does not have internet access?
I have a vanilla installation of ArcGIS Enterprise 10.6, assembled with ArcGIS Enterprise Builder.
If I try and uploaded a CSV file to the ArcGIS Portal which has the data format "building_name, easting, northing," I get this message:
Your portal is not configured to find locations from address stored in files.
I believe this is because ArcGIS enterprise needs a geocoding service accessible and the default ArcGIS Online service is on the internet and we are behind a firewall, so it can't be accessed. Though in this case I have locations already and no addresses, so why is it prompting me to find locations from addresses when there are not any?
To summarise, my question is why do I need a geocoding service to host as a feature layer a data source that is already geocoded? (i.e. has Easting/Northing in a given projection EPSG:27700).