I'm really annoyed with the streaming app situation for the Google TV device in the UK. We don't have access to an app for All 4, Sky's NOW, BT TV, Britbox, UK TV Play - I know most of these can be cast and that Britbox can be available as part of Prime Video but that seems besides the point when other platforms, including Fire TV which runs on a version of Android TV, has these apps available. Just really wish they'd sort it out!
It wasn't that long ago that iPlayer wasn't available on most Android TV boxes. Maybe that's still the case, I'm not sure. It wasn't available as an app to most boxes before the Chromecast with Google TV came out (it was available on Shield, Sony and maybe others, but was a different app for each, not official BBC, I believe). I had a MiBox 3 for a long time and used Kodi or casting for iPlayer access.