You could try writing your own dictionary and then using
that to check your documents instead of spellcheck.  I
don't think there's any way to delete specific words from
the pre-supplied US dictionary.

1.  Turn off the setting, Tools / Language / Set Language, "Detect
Language Automatically".

2.  Make sure that the UK dictionary is set as the default in your
Normal.dot file.

3.  If the above doesn't work, you can create an "exclude" dictionary
and add words to it, that you want to be flagged as incorrect.  The
built-in Word Help usually doesn't explain it very well.  Try these
pages instead:

http://www.mvps.org/word/FAQs/General/ExcludeWordFromDic.htm
http://ccl.english.ohio-state.edu/handouts/miscellaneous/word_custom_tools_pc.htm
http://support.microsoft.com/default.aspx?scid=kb%3Ben-us%3B211306