Hello. I posted a question about this recently in the (unofficial) Fedora forum but have received no replies.

I installed the package named in the heading but, even after several re-boots, British English still does NOT appear in Desktop Settings > Common Appearance and Behaviour > Locale > Country/Region & Language : Languages tab.

Is there any file I can edit or another package I need to install to make British English available ?

do you have /usr/share/locale/en_GB/entry.desktop and does it look anything like a legit desktop service?