UK dictionary doesn't work

I have written several documents without realising that I have had typing mistakes which had not been signalled by the automatic spelling check. I have set the default language to English (UK) and even explicitly set the document’s “All text” language to the same and even intentional spelling mistakes are not signalled. Changing to US English works, though clearly the differences are not acceptable and distracting. I have “completely removed” libreoffice-writer and reinstalled to no effect, and even tried restarting in safe mode with no change so I deduce that the English (UK) library does not wor for some reason.

The documents include job applications and it’s very embarrassing and possibly discounts me from consideration if I seem to be unable to check my documents!

I am using Ubuntu 18.10 (fully updated) with LibreOffice Version:
Build ID: 1:6.1.5-0ubuntu0.18.10.1
CPU threads: 8; OS: Linux 4.18; UI render: default; VCL: gtk3;
Locale: en-GB (en_US.UTF-8); Calc: group threaded

It seems the en_GB dictionary is not installed. You can check if a dictionary is installed or not by looking to the left of the language name on the drop-down list at Tools → Options → Language Settings → Languages → Default Languages for Documents: if there is no icon there (an “A” with dots, an “ABC” with a green check-mark… it depends on your icon theme), the dictionary is missing. Use your package manage to search for myspell dictionaries, for example myspell-en_GB