HDR is all over the place at the moment. Firstly, you need a TV that can pump out at least 600+ nits (ideally 1000+) AND have good black levels, either an OLED if the nits are on the lower side or a fully array LED if they are 800+
MOST tv’s under £1000 won’t give you that and in effect HDR could be making the picture worse. The TV has to also display enough of the REC2020 colour space to display a wider colour gamut too - again, means a higher end set.
then there’s the fact that quite a lot of HDR content doesn’t actually display peak brightness higher than 300-400 nits due to dodgy implementation (check out HDTV Test on YouTube, he’s currently going through a load of 4K discs and uncovering the truth) so you aren’t getting the promised highlights and improved shadow detail, however your tv is whacking its backlight to full.
So currently, on a lot of consumer sets, even good ones, with the wrong source material, SDR is going to look better