Hi
Just to confirm that you reset your YouView remote as per:
http://bt.custhelp.com/app/answers/detail/a_id/49676/~/how-to-reset-your-bt-tv-remote-control
I assume that your JVC TV (these are made by Vestel) has a number of HDMI sockets.
Have you tried connecting your YouView box to another HDMI socket to see if that helps?
If your JVC TV isn't a 4K set, have you tried changing the output on the Ultra HD box to reflect this?
I.e. go to YouView - Settings - TV Signal & Quality and change HDMI Display Setiing to 1080i or 1080p to see if that helps?
Also make sure that your TV has the most up-to-date software installed.