I doubt Sky HDR broadcasts go very bright very often and even then, it's only various small aspects of video images that hit the peak highs of 600-1000 nits. This is even the case in many 4K HDR blu rays. A number of broadcasts/streams, The Mandalorian for example on Disney+ are using fake HDR (using HDR containers to trigger the setting on TVs) and even though this series comes up as Dolby Vision on my LG OLED tv, it doesn't exceed 200 nits for entire episodes. Various scenes still look bright as well as object highlights, but they would on any TV, including @Mackemholic's Samsung UE43CU8500KXXU.
If I were to take a guess, I reckon Sky's HDR programmes don't go above 200-300 nits for 95% of actual broadcasts.
On another note, there are known issues. When iPlayer's His Dark Materials first came out, it was unwatchably dark on my LG OLED tv (which has been calibrated). A Very British Scandal was also the same. I emailed the BBC to let them know and since, they look a lot better but still not perfect.
I have no idea if this is the fault of the TV not handling the HDR image properly OR if it's the fault of the data from BBC, or both. But it's been improved since I and probably many thousands of others told the BBC.
In addition, HDR isn't all about brightness. It has a hugely increased colour palette which would still make content look far more lifelike, so nothing should look dull, even if a TV is only capable of 300 nits. The Mandalorian series doesn't go above 200 nits but it still appears to show a much better colour palette than SDR content so metal, fire, the sun and other known materials/objects etc that SDR has difficulty showing properly do look more realistic.
