Read the "set a default language" section here to get you
started---you'll see that the notion of "loading a UK dictionary" does
not really apply.
http://word.mvps.org/mac/SpellCheck.html
Once you have set text as UK English, try creating an AutoCorrect, and I
think it should only apply when you are typing in UK English. I've not
experimented with this particular one, but in general AutoCorrect is
language-dependent (you can have separate AutoCorrects for English and
Spanish, for instance).