You might be able to use limited Sonos Voice Assistant without an outside Internet connection, but I’m not certain how you would activate your Sonos devices initially. Based on my own experience, I think Sonos has made a recent change, in which they check first for an Internet connection first, which would defeat your aims. 
 

Why can’t you have internet? It kind of defeats the purpose of streaming speakers. Or am I misunderstanding your situation?

Hi Airgetlam

There IS WiFi here but via an LTE WiFi Router, not via BT/Vigin/Sky due to the local set up. 
   Does that help or inform ?

Short answer….

All voice assistants (Alexa, Google, Siri) require internet access with the exception of Sonos Voice. However as ​@Airgetlam mentioned you need internet access to setup Sonos initially. 

Thx

AGAIN, I obv. have WiFi but via an LTE wireless router with a micro sim. I believe that may be some kind of built in protocols to block setting up a voice assistant. Or that was the case 6mth ago. 

I don’t know if there is -or is not- a software update & may have to wait till Monday to ask SONOS Tech. 
 

Cheers, Nick

There are members in the community that use LTE wireless. It’s not the ideal choice (if you can choose otherwise) because as you know providers will throttle your service based upon traffic and/or time of day. If there is a setting to restrict setting up a voice assistant it would not be on the Sonos side. I suggest you contact the LTE provider.