I have a 77" C8. With regard to Sky Q and which resolution is the better setting for HD I wanted to get an example so I downloaded Season 10, Episode 4 of Curb Your Enthusiasm last night. I had the Sky Q box set to 2160p. There is a scene (17 min mark) where the cast are exiting a plane onto the runway. If you watch the character Susie Greene, she is wearing a red and white striped top. I see jaggies and bleed when watching this scene in 2160p. If I switch to 1080i this is cleaned up by the C8's upscaling. It looks perfect with no jaggies or bleed.
This test scene has also been confirmed to work by
@Andrew1472 here:
LG OLED C9 Owners and Discussion Thread
It might be that some Sky Q boxes are better at upscaling than others? I can see that my C8 is better at it than 'my' Sky Q box.
Update: it appears that it is the progressive sstting that is causing the issue. As pointed out by
@Parry in the C9 thread the scene above is not affected when set to 1080i but is affected when set to 1080p and 2160p.