Use Siri's voice as the system voice

I notice that on Sierra, Siri's voice is much superior to the selection of system voices you can select in System Preferences → Accessibility → Speech. In fact, I suspect they use a different system entirely as the command say -v ? doesn't seem to list any voices which are used with Siri. I have a key command set up to speak the select text. Is there no way I can get it to use Siri's voice to do it? **Edit:** To be specific, I'm after the Siri voice: 'British (Male)'