I have a Samsung 4k TV and BT 4K box. When there is 4k content on BT Ultimate we get a green button prompt to switch to Ultimate, but with the BBC coverage of Euros there is no prompt to switch to 4K on the iPlayer. This probably is a BBC thing,but would be great to have a prompt similar to BT.. I also noticed today when you press Red on BBC channels I get a different home screen for BBC connected Red Button..
I think it is a BBC issue.
The BBC are only ones showing matches in 4k on iplayer.
To watch them you'll need an iplayer account and also ensure that in iplayer settings you have the iplayer beta/experimental settings flag switched on.
My BT box is only hd so I use iplayer on my Sony Android 4k TV. The iplayer on that 'knows' it is on a 4k tv and gives me prompt to watch in uhd. Picture and sound quality is excellent compared to HD. In fact when going back to hd it looks like old 720hd lol
My Samsung TV gave me a prompt for the tennis but not had one for football in UHD.. I have a BBC account for iPlayer,etc. But the BT 4k box doesn't give any prompt when watching through the BT box.