I guess you didn't do a Forum Search before posting ? There are lots of people who have reported this.
It is all to do with HDCP (Google it if you don't know what it is). Your YouView box will only allow the component video to work if it gets a valid handshake from the device on the end of the HDMI cable, in your case the TV. The solution is to remove the HDMI lead completely and feed your TV from the Component OUT of the Slingbox.