Please be aware - Amazon have changed the Group settings and “Preferred Speakers” are no more.
Using an Alexa Device Group to get over the BBC dropping TuneIn problem can still work though.
Create your Device Group and put your Sonos speaker(s) and the local Alexa device into it. Ensure the Sonos speaker(s) are set as the “Speakers” for the group.
BE SURE TO SCROLL RIGHT DOWN THE SPEAKER LIST to find the “Playback Options”.
Chose “Change” and select “Always”