Sonos cannot do this unilaterally, either legally or technically.  it is for the BBC to use the Sonos API to integrate BBC Sounds, although it is of course to some extent a co-operative venture.  That is what Spotify did.

It is also clear that the BBC likes to keep control of the distribution of their content, hence their withdrawal from Tune In.

it is questionable whether, as a publicly-funded organisation, the BBC should be devoting resource to developments with a particular speaker manufacturer.

You really need to contact the BBC about this.

I agree. Easier said than done however. As for the BBC’s resource allocation decisions, I guess it depends on how much BBC media is being delivered by Sonos. I expect that is quite a lot. There was such a hoo-ha when the BBC radio stations disappeared off TuneIn they reappeared immediately. 

Additionally I guess it would be difficult to control licensed content, or the Beeb haven’t worked that out yet.

on the other side of the coin why is the Beeb wasting money making something work on a single supplier. It is up to the likes of Amazon and Google to make this work with their assistants instead of blaming the BBC.

I assume that TuneIn caved into the BBC’s request - the Beeb wanted listening data, anonymised, for their planning. The main complaints were from e.g. Alexa users who were then deprived of using a Beeb station in an alarm.

Absolutely. Spotify’s licensing issues are far deeper, and they seem to have managed.