as they say "you can't polish a turd", and apart from the HD channels (and BBC SD channels) you'll find that the picture quality of the SD channels on a large 4K TV is pretty bad.
that's the not the fault of the box or the TV, just the the bitrate of the channels is low, and they are SD and you are essentially upscaling them to 4K resolution. all that does it make an already bad image even worse.