Hairy Scot said

I have language set to English (UK) but spellcheck still uses English (US).

Hello Hairy Scot,

Would you please check if you've got a British English Dictionary installed :

3-bar menu => Add-ons => Dictionaries

If not; maybe you'd like to download one of these :

https://addons.mozilla.org/en-US/firefox/addon/british-english-dictionary-2/

https://addons.mozilla.org/en-US/firefox/addon/british-english-dictionary/

(edit : typo .... )

Note that the language menu is about the language used for the strings in the user interface and not for a spell check dictionary. Installed dictionaries should appear under the Dictionaries tab on the about:addons page.

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

Note that you need a dictionary and not a language pack that provides string translations for the user interface.