I am having the same problem, I recently got a 4k TV 65inch , the sports channels are terrible blurry faces unless zoomed in bt sport slightly better but still not hd ,and the bt uhd channel is hd at best , i get that the 4k will only work with compatible channels but it shouldn't turn my hd channels into sd ,i spent 2 hours on with virgin to be just past from pillar to post , I ended up upgrading to the new 360 boxes what have arrived but just the same blurriness , netflix prime are perfect , i was blaming the TV at 1st but for 3 weeks I have been trying to figure out what to do ,I've tried every setting combination on both my tv and virgin box and no joy , absolutely fuming with virgin and the poor quality picture ,
i know this isn't a solution but after reading your post it made me feel better that I'm not going mad and there is an issue with the box,
hopefully we can find someone who will understand and not tell us were doing something wrong or tell us to switch it off and back on 🤣