I've confirmed my Britbox tvOS 'softness' feel tonight. Playing content I already own via iTunes with the same material on Britbox. Britbox is inferior to the ITunes product BUT not a game changer. I await some content that recent and that I own to appear that's rich in Colour and Contrast to give a full assessment. This Judgement so far is based on the 'Nature's Great Events' series that's ten years old now and was a very early HD production, and was transmitted on the BBC HD Channel in the day, and might be hampered by that.
Judgement still out.
If anyone has the Apple TV app for Britbox, then if they have a Mac and Xcode (which is free) installed on it they can enabled Developer mode on the ATV which allows you to enable a playback HUD (an OSD) which tells you the resolution, frame rate and bit rate of the video stream being played.