If you don't have the voice installed already, on Windows 10 version 1809 and earlier, you'll need to download the English United Kingdom language pack which includes the voice. You can do this by going in to settings>time and language>language, pressing the add a language button, and selecting English, United Kingdom from the list. Press enter on the language, click next and follow the prompts. On Windows 10 version 1903, you won't need to do this, you can just download the voices on their own.