Enhanced player - you can only choose the picture quality when you launch the Enhanced player, not when you're already in it. So if you want to change, you'll need to close the player and re-open it.
Standard player - you can choose the quality within the Standard player. Simply click on the 'HD/SD' symbol while you're watching and pick the one you want.
I’m glad that it’s not just me that has had this problem. I first noticed it about a month ago and reported it on Twitter. I have a Chromecast Mk 1 & 2 and it doesn’t seem to make any difference if I swap them over. I have exactly the same issue, sometimes BT Sport1, 2 & 3 will be fine but BT Sport ESPN will have a problem. I wondered if it was because I had plugged the Chromecast into the USB slot on the tv rather than the mains, but that doesn’t seem to make any difference either. I watched loads of sport over the weekend without any issues. I’m going to watch the Scottish football this evening, but at the moment it isn’t casting in HD. It’s extremely frustrating. It’s almost as though someone has switched over to the picture quality that we had before it was upgraded to full HD back in January. So, it isn’t just me and my set up, there is most definitely something going wrong here.
Ok, that’s interesting. I don’t have BTTV. I just pay for the BT Sport app/online. It’s just becoming more and more frustrating. I’m watching at the moment and BT Sport 2’s in HD but all of the others aren’t. It doesn’t seem to matter what I do. I have no control over the quality of the picture when casting. I don’t think that the problem is with the app myself, because the channels are all in HD on my iPad. It’s only when I cast them that the problems occur. I think that the problem is with Chromecast, but it does seem bizarre that at any given moment one or more of the channels are in HD when being cast, but the others are not.