A family of Microsoft word processing software products for creating web, email, and print documents.
If you type into Word from the keyboard into a new blank document, the proofing language is that defined in Windows as the default input language. If that isn't happening you don't have UK English set as your default input language.
If you copy text from elsewhere, especially the internet or from other people's documents, or create documents from other people's templates, then the proofing language applied to the source text will be copied with it, or as defined by the paragraph styles of that template.
Every character in a document can have its own separate proofing language.
The language button, optionally available in the Word status bar, will indicate the language at the cursor. Click that language button and ensure Detect Language Automatically is unchecked.
If necessary, you can select the whole document (CTRL+A) and apply the required language to the selection using the aforementioned language button.