I have a 4k/HDR TV: an LG model CX.
On switching to a Euros match on FreeView BBC, an icon appears in the
top right hand corner advising that to see the programme in UHD on the
iPlayer, press Red. On switching, an HDR icon appears briefly in the
top right hand corner.
In the iPlayer settings you can choose between 24MBit/s 3840 pixels or
12 MBit/s 2560 pixels.