What you need to do is select the content, use Tools> Language to select English UK. The Default determines what the Language formatting is for the new documents you create. Open a document containing text formatted as a different language that ttext will not be checked using UK tools. You can reset the 'default' as much as you like, but any time you If the text is tagged as English UK, Word uses those tools.
I.e., if the text is tagged as English US, that's the set of tools Word uses. Is determined by the Language formatting of the Which set of tools it uses for any given body of text, however,
Word's spelling checker uses all of the installed proofing tools for each document. You cannot change the 'default spellcheck' but there is no reason to.
