[wrxtasy]

In general I just want to watch HD content (I guess 4k will be a good investment for the future since there is not much available now but will be in the future and I got the TV) with 5.1 audio. Any similar hardware that I can find in Amazon.uk is fine too.

Any advice would be very much appreciated.

You already have Hardware for Netflix and Amazon so avoid Android all together. Run Kodi without compromises, use LibreELEC.
Especially for Auto Resolution switching 1080p <<-->>2160p(4K)

Simply get a AML S905(x) box like everyone else and run LibreELEC Kodi Krypton on it. Some come with Wireless remotes.
Otherwise buy something like a MINIX A2 Lite Wireless remote.

Eg:
S905 MINIX U1
S905 WeTek Hub and Play2 then buy a Wireless remote.
S905(x) Vero 4K

4K Netflix and Amazon = FireTV2 or nVIDIA Shield. You need a Dolby Digital Plus Audio receiver (TV / Soundbar) to get Netflix and Amazon 5.1 Audio out of the Shield. Amazon FireTV2 has better Audio handling for these Apps due to having an actual Dolby Audio licence.

I believe the FireTV2 can do 5.1 Audio just fine with Kodi. HD Audio is decoded to Multichannel PCM.

I do not recommend cheap AMLogic boxes for Kodi usage. Those generic Android boxes are NOT Android TV devices.
Android TV = Shield and Mi Box + Sony/Philips TV's.

Touchscreen Android Apps usage is a PITA with an Air mouse remote on generic Android Boxes. Not recommended.

Use Android for Android Apps only.