Most BBC main radio stations have disappeared from the radio browser - both in the UK and elsewhere. There was some news about the BBC stopping streaming stations outside of the UK, but unless the Radio Browser is hosted outside of the UK, it shouldn’t have affected the browser in the UK.
There is an open issue in GitHub on this issue (which I am also experiencing) https://github.com/home-assistant/core/issues/134262
The BBC HLS streams work in the TuneIn app (not sure if Media Browser uses HLS streams though).
I’ve just tried that… and I get an error - Sonos does not support media content type: audio/mpeg - despite the other play scripts I have for other stations (through Radio Browser) working. Did you need to change anything else? My script has multiple references to media content id - this is one that still works: