you've probably either worked this out or given up by now, but as I had exactly the same question I thought I should post my solution. (I'm also a newcomer!)
The issue was that in my pre-packaged ubuntu install, it didn't actually install the UK English dictionary, so although the language was set to UK English, there was no dictionary to check against. In the Tools -- Language Menu there's an option "More dictionaries online". This takes you to Libre Office website. You can then type in "English" in the search box that's offered, and scroll down until you see a pack that has various different English spelling dictionaries. Click on it and you get a page, and in this a link to download a .oxt file.
I did this, and it offered me the option to open this with Libre Office, which I did, and this installed the dictionaries. UK English then started up OK -- and also South African, Canadian and a host of other version of English become enabled, too. You can see that they are there, because they have a little "abc"-tick icon next to the language.
Hope that helps others with the same problem!