No Youview box needed if planning on using BT Sport via the app or the browser. The main problems with only having access that way though is lack of recordability and the difficulty of avoiding spoilers when wanting to watch some sports coverage on catch up.
The interface was obviously designed by someone who is not actually interested in sport and doesn't understand the problem despite having it explained to them over and over. That is why I prefer to record using a Youview PVR.
BT Infinity 1 (unlimited)