How exactly are you looking for them? Using BBC Sounds, a direct url, or an aggregator?

BBC Sounds

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 ostation 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.