Glad there is a PQ improvement for you by forcing 1080p on your 4K Roku when watching NOW live Sky Sports.
I have tried this in the past on numerous occasions with various devices and for me the camouflage picture artefacts are still there for me when watching live Sky Sports.
Thought I would give it another go after seeing this post of yours, unfortunately it is still the same for me 😡 .
What I have noticed in my setup when comparing two different 4K televisions is that my OLED 10 bit TV seems to mask the camouflage artefacts better (still noticeable to my eyes) when comparing to an 8 bit mid range LCD television that I own where the camouflage artefacts is more pronounced.
Anyway glad it solved it for you.