It's possibly due to the actual TV. Some of the older and/or less expensive TV's don't have a bright enough peak brightness to work with the type of HDR that TNT and SKY use which is called HLG so the picture can appear washed out/dimmer. Ideally the TV needs a peak brightness of at least 600 nits (aka cd/m2) Streaming apps such Netflix, Disney+ and Amazon Prime don't use this type of HDR and so aren't prone to as much dimming.
What does the TV picture look like when selecting UHD using it's inbuilt iPlayer? Is there any difference using the V6 boxes iPlayer in UHD?
You might be able to play around with the picture mode settings such as Vivid or Sports on the TV to achieve a better balance, or delve into some hidden picture settings (try googling) on the TV such as those found on my LG TV (see <Here> ) and maybe set the picture to HLG or SDR to see if that improves it.
A quick google shows, "Hidden picture settings on a Hisense TV can be found by accessing the Picture menu and navigating through submenus like General and Advanced Settings. Key "hidden" or often-overlooked settings include disabling features like Light Sensor and Intelligent Scene for more accurate picture quality, especially in HDR content. You can also adjust settings for specific use cases like Game Mode to reduce input lag."