Comment 1 for bug 756292

I had the same problem on Mint 17 KDE. The solution I used was to install the dictionary from:

http://extensions.libreoffice.org/extension-center/american-british-canadian-spelling-hyphen-thesaurus-dictionaries

as it's not included as a package in the repositories. Having to do this is a bit poor given the intended user base.

Others might also want to look at the packages:

libreoffice-l10n-en-gb
libreoffice-help-en-gb
libreoffice-lightproof-en

(replace gb with your country code as appropriate). None of these packages seem to be installed by default either.