Yes, thank you very much Hardy! Perhaps I should look no further.
Your link has a very good collection of (almost) what I was looking for with some reservations.
These are not
authentic keyboard scores but who cares if one can get either these still authentic 2-part Renaissance counterpoint pieces or otherwise what you call
the flower of English keyboard music which is not for everyone's chops.
Anyway, what basic repertoire do you (or anyone else) think was available for early keyboard amateur players of the 16-th century? I suppose not everyone was ready for the flowers of John Bull and William Byrd?
I mean if we know that it makes the whole task easier: for instance the majority of 16-th cnt. amateurs could indeed play intabulations of some two-part chansons and other vocal pieces most of the time.
What else comes to mind?
Basically what was possibly available in that period as a solo instrumental music can be summarized in just a few common forms as I only recently found:
- Dance music (Pavan, Galliard, Alman, Basse Dance). These are unlikely to be in "clean" counterpoint and the Fitzwilliam shows they are not.
- Intabulations (as you correctly noticed) of the vocal reductions. Your link provied a good example of this texture and it's a clean counterpoint. Almost academic

Which is good!
- Ground Bass variations. This is probably a good form to build up Renaissance chops having improvised music in mind based on diminution but that's not exactly counterpoint or... is it? Probably Ground Bass can be considered an ostinato "cantus firmus" in free terms but I don't think it's a good practice to make up extentions of the commonly accepted terms.
- Abstract instrumental works: Ricercar, Fantasia and Canzona (coming from 'Chanson') and those are a plenty in the Fitzwilliam book but they are beyond my modest playing abilities. The
flowers - indeed they are!
You are looking at the flower of English keyboard music! Try this instead: https://home.planet.nl/~teuli049/JosqDuo0Collectie.pdf or search for his keyboard intabulations (though I think that link may include them).