Yes, if you have set your Sky Q to 2160p (Ultra HD), you should be able to watch HD content on the Discovery+ app. Discovery+ supports streaming in HD quality, and with the Sky Q box set to Ultra HD, you will receive the best possible quality that the app offers. Just make sure that the content you're trying to watch is available in HD or Ultra HD, as not all content on Discovery+ is available in the highest resolution. Enjoy your streaming!