Each Word Document inherits (and 'remembers') which Dictionary to use.
So even if you have English (U.K.) set as default for new Word documents this will be ignored if you open old documents or documents you have received from other people.
Just to make it even more interesting the dictionary can vary paragraph by paragraph in one document!
Here is a quick fix to ensure your whole Word Document is using one Dictionary
1. Select All (Ctrl+A)
2. Click the Review Tab and select the Language button
(In earlier versions of Word select Tools / Language / Set Language (or double click Language in status bar along the bottom)
3. Select English (U.K.)
4. Click OK
Tip - To make sure any new Word documents you create use the English (U.K.) dictionary - carry out steps 1, 2 and 3 above then click the Default button.