Aran

It was more to clarify the need for Serif (and myself).
Hope you get your wish. 🙂

Photoline
Hey Medieval Officer Bones,
thank you very much for the recommendations! 
unfortunately Photoline is too expensive for that single feature, as basically I am happy with Affinity, just missing the above described feature at the moment.
I've checked out Krita, and that seems to do the job, TYVM
With beste regards
-Aran

Another option is PhotoLine that runs on Mac (M1 native too).

Not as controllable as CQ, but an added benefit is that PhotoLine supports a non-destructive approach if required.
Or Krita's Palettise filter, which again may be applied non-destructively:

Krita offers a really nice option to control the dither pattern as well as dither range. Also works on Mac, of course.
Both reduced to a 27 colour Amstrad colour palette
Actually, most image editors feature a similar colour palette reduction option. I am starting to wonder why Affinity Photo still lacks one. Odd. After all, such a basic tool? Then again, so many other basic things are still missing, so I shouldn't be surprised.
Perhaps in release 2 or 3?

As far as I understand a LUT is more like a 'look' one can apply to an image. What I am seeking is a recacluation or even reduction of an image to a certain set amount of defined colors - I come from Photoshop and there was the option of creating an indexed color palette from eg self-determined colors and the one could apply this indexed color range to any image. Since I quit using PS I cannot make screenshots or a vid of what I mean in detail.
Thank you very much for the explanation and video 🙏
That helped me understand the swatches panel a bit more, now I can create own color palettes and even extract the colors from an image.
Still I don't get the difference between the program-, document- and systempalette. That is all the same if applied to a singe document or as a modus operandi for batch editing and as a global preference.
Still I would need to apply the created color palette to an image rather than from an image, there seems to be no option alike --- therefore the suggestion for implementing this option (or maybe there is actually a workaround?!)

Hi @Aran
If I understand right, you would have a specific palette that represents the colours from e.g. a Company?
If so, you can create your own colour palettes, either as a programme palette or as a document palette.
And indexed is also possible by creating the colour palette from the document or from an existing file. Again, for the programme or for a document.

If you are asking for what I think you are asking for then it sounds like a LUT (Look-Up Table) might be what you need.
If a LUT won’t do what you want then more explanation might be needed, maybe with some workflow examples.