why cant hey sonos find BBC radio stations
Because BBC Sounds is not compatible with Sonos Voice Assistant. It is compatible with Sonos Radio which it should default to - but BBC Radio stations are not supported on TuneIn Radio which Sonos Radio uses.
I see you have Alexa. You would need to say “Alexa, play BBC Radio [station name]” for it to play on your Sonos speakers - provided 1) you have linked your BBC account to Alexa in the Alexa app, and 2) you have added BBC Sounds as a service to the Sonos app.