You need to ask someone running Tiger to check this out. Just experimenting, I can tell you that after changing my regional settings and language to UK "colour" was still spelt color in Finder; I didn't check other applications. This was in Panther.
Apple has steadily improved language support in each release of OS X, and I'd expect British localizations will actually exist in Tiger. The universal spellchecker in OS X does recognize British English however.