It's disappointing, but remember beta testers are under NDA and aren't supposed to discuss the betas with the wider world. However, people have been talking for a while about how AirPlay Mirroring would work based on what Apple has said/shown, and the limitations were always obvious (i.e. we always knew it would be a 4:3 image, and the demands of real-time video compression always meant that video quality was going to suffer).
Things should get a bit better if/when the app developers take AirPlay Mirroring into account and code for 720p output.
You can still get iPlayer on your TV if you AirPlay from Safari on an iPhone, if that helps (though apparently not from an iPad).