Your Denon X8500H does support HDMI 2.1 4k/120 and eARC, not sure why you are saying it doesn't? Is it different specs in the UK?
Scratch that, the x8500HA does support 2.1. You can upgrade the Denon, but it is $599.00(US)
ARC can support passthrough Dolby Atmos, though not guarenteed depenent on devices.
Denon upgraded my AV8802 to the (A) Designation for free to get HDMI 2.0, not happy they the charge for it now. But I am sure its expensive