My guess is that the signal strength isn't as good as it needs to be. (A stronger signal is needed to resolve HD transmission than SD ones. A weak signal can lead to picture break-up).
If you're using a satellite dish, you might need to get its alignment checked, together with its cables and connections.
If you're using Freeview, try one of these
https://www.argos.co.uk/product/9383027
together with one of these (to connect between the booster and your TV set)
https://www.screwfix.com/p/philex-coaxial-cable-1m/41720