BT YouView boxes do not support WiFi at all. It's not considered reliable enough for the sort of live streaming channels that BT uses.
The alternative is powerline adaptors. They are still not officially supported bty BT, and might not work on the HD channels.
For most people, powerline adaptors work fine for SD and HD. Maybe even UltraHD. But the speed you get through them depends on your home electrical wiring, so BT avoid telling you that they will definitely work.