I watched the first episode of Blue Planet 2 last night in 4K HLG via my 65" LG OLED. Truly breathtaking stuff, by some margin the best streamed 4K content I've seen. Obviously no match for UHD BD, but it would be unfair to expect that.
The bitrate is 28mb, which is higher than both iTunes 4K and Netflix 4K, and the HLG HDR looked beautiful. There was some banding on extremely dark scenes, but it was pretty minimal.
Compared to regular 720p 5mbit iPlayer it's truly a revelation. The only fly in the ointment was the lack of 5.1 audio. Sadly it's just the usual 2.0 channel stereo.
Shame it's limited to just smart TVs, but it's early days yet. I don't believe many 4K boxes, such as the PS4 Pro, Xbox One X, ATV, Fire TV, and Shield actually support HLG, so who knows how the BBC will solve that one.