Hi @enoemos
Welcome to the Sonos Community!
Currently, Alexa is the best option for using BBC Sounds with voice commands. First, please remove Google Assistant from your speaker(s), then add Alexa back on. Sonos app » Settings » Services & Voice » Google Assistant » [each room in turn] » Remove Google Assistant. Then, from Services & Voice » Add a Voice Assistant » Amazon Alexa and follow the on-screen prompts.
Next, disable the BBC Sounds and Sonos skills from within the Alexa app, then add them back on again. Alexa app » More » Skills & Games » scroll down to and tap Your Skills » Sonos/BBC Sounds » Disable Skill then Enable to Use.
Please then remove the BBC Sounds service from your Sonos system, and then add it back on again. Sonos app » Settings » Services & Voice » BBC Sounds » Remove Account, then in Services & Voice » Add a Service » BBC Sounds » Add to Sonos.
This all should have refreshed the security tokens exchanged between the involved services to a degree that they should now work together again. If it has not, I recommend you get in touch with our technical support team, who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.
I hope this helps.