In my Office Proofing (and Outlook), the default language and dictionary are set to English UK, but when I start an email, the default spell check is English US.

Hi,

In your scenario, please make sure the Default Proofing Language has been set to English UK, and untick "Detect language automatically" to check the result.

You can find the settings when composing a new message, Review tab -> Language group -> Language button -> Set Proofing Language. See the image below (my default Proofing Language is English (United States)):

To set the Default Proofing Language, select the language you want and click "Set As Default" button.

Regards,

Melon Chen
TechNet Community Su