Ran into same problem. Even though pay for BT sports you cannot login into BT Apple TV app unless you pay an additional monthly fee for App extra feature. Total con in my opinion. Just end up mirroring it through airplay to Apple TV instead.
I’ve asked this on the other BT Sport thread too, but how will the HDR work? Does the ATV 4K support HLG as that is the only HDR format that works for live programming. Can’t find any info.
I've found this app takes on the UI display settings and doesn't force the 1080p 50Hz that matching source should.
On the latest beta so not sure if that makes any difference.