Chloropleth in Easting, Northing


I’m trying to make a chloropleth map based on UK counties and so have a GeoJSON file which is in the OS grid system of eastings and northings.

I’ve tried to use this file to create the map but I get huge errors which I presume is because the default map in plotly is in longitude and latitude. Is there a way to create a chloropleth using the eastings/northings without converting to long/lat?