Open Office installs with the US dictionary as the default language, if you want the UK version, you'll have to download and install it for yourself. I didn't find this as intuitive as I would have liked: mostly because Open Office uses ambiguous titles for its extensions making it difficult to locate them online. However, I recently did some digging and I have managed to find the correct download and now I have a working (and workable) UK dictionary and thesaurus.
The file that you want is the English spelling and hyphenation dictionaries and thesaurus: don't bother searching, it's a nightmare to find! You can download it from here (the file is actually called dict-en.oxt, but, for some obscure reason, the title on the Open Office website is English dictionaries with fixed dash handling and new ligature and phonetic suggestion support).
If you're using Open Office 3:
- Download the file from the link above and save it to your hard drive
- Open the Extension Manager (from the Tools Menu) and click Add...
- Navigate to your download from the dialog box and click Open to install your dictionary
- From the Tools Menu, click Options...
- Under Language Settings, select Languages and set the Default languages for documents option to English UK in the Western dropdown menu
- Under Writing Aids, make sure that the Spellchecker, Hyphenator, and New Thesaurus check boxes are all ticked
- Restart Open Office