You can 'cast' video to a Chromecast from either the iOS or Android version of the BT Sport app.
First, set up your Chromecast according to the manufacturer's instructions and put your TV on the correct HDMI channel.
If the device running the BT Sport app is on the same Wifi network as a fully set-up Chromecast, a casting icon appears in the top right-hand corner of the app home screen.
When the user taps the casting icon, a pop-up window appears displaying all Chromecast devices detected. The user needs to select the Chromecast that they wish to cast to, regardless of whether they only have a single device.
Once the device is connected to the Chromecast, the casting icon in the top right corner is illuminated blue
When the casting icon is illuminated, it does NOT mean that the page is mirroring to your TV. It simply means that, when you do start playing a video on the app, it will cast to your TV.
A viewer can also start casting while a video is already playing within the app, by following exactly the same process.
When you are casting from the BT Sport app to your television, the video will not also appear on the screen of your device.
If you are having problems streaming the BT Sport channels to your Chromecast, it is recommended that you try using the extender that is included in the Chromecast packaging, as it may be that the Chromecast is not receiving a strong enough Wifi signal from your router.
An update on Sean's post on airplay/casting to your TV using an AppleTV or Chromecast - we've recently released some new 'native' clients (a.k.a. 'Large Screen App') for Apple TV, XBox, and Samsung TVs (2015 models & onwards).
You can download the new 'Large Screen' BT Sport App from the equivalent App Stores on each of those platforms and view the live streams in a slick new interface.
There's more info here. We're working hard on other new features for those Large Screen Apps and on additional platforms.