If you google now tv streaming quality and follow the link to the Now TV help page you will see the the quality that you might get on various devices. Note that apart from on Youview which is a fixed SD quality all the other devices have adaptive streaming so the actual quality you receive could vary significantly and the quoted figures are the maximum achievable,
Personally if I watch Now TV I do so on Apple TV.
Remember that NOW TV Sports on AppleTV are transmitted at 720p. What you are getting is the SD service upscaled to that, not a downscaled HD service.