Why does my Word 2007 spellchecker default back to English (United States)

If I copy a section of text from a web page such as this one and post it into a Word ducument, the language is set to the US version of English. So when I want to amend this to the UK version of our language I find that the spellchecker always defaults back to English (US). I can change the Word default language and remove the US version and save the changes but when I try to spell chack the text I find it is again accepting American spelling, and when I check the language from the spell checker pop up box it has always reverted to US English. Help!!!