I wasn’t able to easily download ONS output area lookups from either their main site or the geoportal. I kept getting errors that the download failed. I suspect on Linux the URLs are considered malformed; they might work on Windows.
Anyway, on Linux use links from the geoportal. They directly link to the .zip files (and the main site links to the geoportal, anyway).
I used cURL on the command line and escaped the brackets in the URL with
curl -o ~/Downloads/oa-lookup.zip https://geoportal.statistics.gov.uk/Docs/Lookups/Output_areas_\(2011\)_to_lower_layer_super_output_areas_\(2011\)_to_middle_layer_super_output_areas_\(2011\)_to_local_authority_districts_\(2011\)_E+W_lookup.zip
Which worked a treat.