Netflix: Most of their content seems to be their own TV content
Amazon: Most of their own TV content seems to be available in UHD, and included with Prime. Most if not all movies available in UHD are paid for rental/purchases.
Apple TV: From what I've seen has a good selection of UHD and HDR content for TV and Movies.
Sky Q: Mainly sports and download Movies from what I've seen. Requires the 2TB box and a £12 p/m supplement to your sub. Not HDR or Atmos yet I don't think.
BT TV: As far as I know it's just sport and some downloads in UHD.
BBC iPlayer: As far as I know, this is only available via the TV's built in apps and some of the Roku sticks. HDR is delivered through HLG, so is not compatible with all TV's if they don't support HLG.
Google Play Movies: As far as I know, this is movies only at the moment. Only works via a Chromecast Ultra and some Android TV devices.