Id bet 95% chance it is compression in whatever you are watching causing it. If the stream is only lets say 8mbps at 1080p, it can handle normal stuff fine. Fast panning, large stadium crowds, rough water surfaces, and other high contrast/detail and/or fast changing scenes overwhelm the amount of bandwidth available, be it cable, streaming, or satellite. Video codecs typically just worry about changes in a scene, so a calm cloudless blue sky scene uses way less bandwidth than a scene with flashing police lights where the entire scene keeps rapidly changing back and forth between red and blue.
A bluray or computer connected to that same TV will very likely produce a nearly perfect picture because those are not bandwidth limited.
I suppose low signal strength or other issues could be a cause too, but I still say its your content itself.
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I have a Sharp 4k, and while it is a smart TV, I do not use the smart features as I have mine connected to a 4k bluray, pc(s), etc. and have no need - any of those devices are more powerful than what is on the tv.
Not that it is terribly likely to matter to Mrs. Peel, but a lot of basic (cheaper) tvs have fewer connection options like only 2 HDMI, which could somewhat limit future add-ons without a splitter.
If I were suggesting a new TV, I would be looking for at least a local dimming/QLED TV and avoid the cheapest of the cheap TVs.