I would suspect that you have things incorrectly wired.
I would interpret the correct way as YouView box to TV, then from the TV HDMI, to the soundbar.
This is my best guess, as its not my area of expertise.
I have a LG audio system, and TV. The YouView box goes to the TV, and the TV connects to the LG audio system. The TV outputs the audio on the ARC channel, to the LG sound system, and it works just fine.
If your TV has only one HDMI connection, then you would have to use a HDMI splitter from the YouView box, and feed the TV and soundbar from it.