The BT YouView box should be plugged into an HDMI socket on the back of your TV. The Wii should plug into a second HDMI socket on the back of your TV. You therefore switch to the Wii by switching input on the TV using the TV's remote control. It isn't about channels. The Wii won't be on any of the YouView boxes channels.
If you only have one HDMI socket on your TV you will have to either get an HDMI switch to allow you to switch between the two or physically unplug the YouView box and plug in the Wii when you want to use it.