AuthorIT provides the ability to import a word list in the AuthorIT Administrator. A good UK English word list is available for Open Office from http://www.pyxidium.co.uk/dictionary/OOo.html.
The file you need from that page is the "Full word list as used in the British English dictionary" en-GB-wlist.zip.
The word list will import as is, but due to the large number of foreign characters, a large number of the words will be rejected. To fix this you need to modify the character list map file.
You can either do it your self using the sample DICT25.MAP provided by AuthorIT, or use the one that I created (dict25.map). Unzip GB-wlist.zip to a folder on your C: drive, and copy en-gb.map there as well.
Now use the AuthorIT Administrator to import these NEW files. Form the Maintenance Menu select -> "Create Dictionary from Word List".
Browse to select the new Word List (en-GB-wlist.txt) and Character list
(DICT25.MAP ). I entered "English UK" for the language. Press Build.
There were a few errors (about 70?), but these were insignificant compared
to the huge number of words imported.
Let me know how you go in the feedback section below!