It’s not just BBC Sounds, it’s everything so far as I can see. I’ve used the Spotify and Pocketcasts services as well and I can’t find those fast forward/rewind buttons on any of them.  I also can’t find anything in settings that would allow the customising of that screen.  It’s very annoying and I’m hoping I’ve just missed a setting somewhere because otherwise if you just want to rewind a minute or so, you just have to drag the bar and hope you’ve got it right.  It was much easier with the dedicated buttons. 
 

I contacted Sonos support about this yesterday and was told that this function has been removed from the app.  This is highly annoying and inconvenient. I listen to a lot of podcasts and audiobooks. The ability to easily rewind by a small amount of time is important. It’s nowhere near as easy to do that by scrubbing back through the podcast timeline. As far as I understand it, there are no plans to address this in any upcoming app release. 
Im surprised more people haven’t mentioned it to be honest. 

Missing this feature is really annoying - Sonos, please reinstate this soonest.