It looks like Twenty Fourteen hasn't been translated for en_UK yet.

You can go to the following link and once signed in with your wordpress.org username you can add the missing translations, then once approved by one of the translation validators they should then be delivered via WordPress automatic updates to your install.

http://translate.wordpress.org/projects/wp/dev/twentyfourteen/en-gb/default

If you want to download the .mo & .po translations manually have a look at the links in the install section on the en_AU page and substitute en-au in the links with en-uk.