Changing Spell-Check Dictionary
By default, LibreOffice only contains a spell-check dictionary in USA English. Attempting to change 'Locale setting' and 'Default Languages for Documents' in Tools>Options>Language Settings>Languages is insufficient to alter the spell-checking dictionaries. To achieve this one must either add the UK English dictionary via Tools>Options>Language Settings>Writing Aids>'Get more dictionaries online...' or for a quick fix (without having to navigate an online repository), simply install the myspell package, by inputting the following line to a Terminal window:
sudo apt-get install myspell-en-gb
after which a computer restart is required to update the dictionary. Upon restarting and opening LibreOffice, UK English spell-check will occur automatically; if not, double check that all language settings in Tools>Options>Language Settings>Languages are set to English (UK) (except for 'User interface' which is fixed on USA).