I noticed recently that LibreOffice was not checking my spelling. I solved the problem as I've done before by
installing an Open Office British English dictionary. After an update to LibreOffice 4 in Debian Jessie, the spell-checker stopped working, and I had a look for a better solution.
The problem seems to be that LibreOffice says it's using Hunspell, but there is no British English dictionary available for Hunspell: you have to use Myspell.
In addition, the British English package for LibreOffice had not been installed by default, so I installed that as well.
These are the packages required:
libreoffice-l10n-en-gb
myspell-en-gb
Here are some screenshots illustrating that LibreOffice clearly thinks spelling is handled by Hunspell when it isn't.