Are you able to tell how much bandwith your using for 4K or HD streams?
Hmm, might be hard when you dont know what your looking for and im not familiar with your router.
But it should be on your interface statistics for your IPTV WAN link.
What you'd need to do is turn on your UltraHD box to an HD channel like discovery or BT Sport1HD then see what bandwidth it uses, then do the same when you change to a 4K channel like BT Sport UHD.
I am at work at them moment so will have a look for you later on and hopefully get info that you are asking for
Another option might be to run a broadband speed test on a computer (preferably connected via Ethernet or at least in the same room as the router if using WiFi) with the STB tuned to BBC1 and then with it tuned to BT Sport 4K. Ensure no recordings are running and that nothing else is using the Internet heavily (e.g. a family member using YouTube or NetFlix).
The difference in speed tests results should indicate how much data is being used for the 4K stream (as the IPTV multicast packets are prioritised over the speed test).
I'd recommend running multiple speed tests with/without the 4K stream if you try this to gain some confidence in the results.