Metal roofing thing (idk how to translate, am not british) are always in mm. We would buy them at like 5 meters long and measure for cutting using mm. https://limarija-sebastijan.hr/krovni-zidni-i-ostali-profilirani-limovi/trapezni-lim/ All the drawings for metal things are in mm. What is not is like diameter of the pipes. The store I linked sells rolls of sheet metal (again don't know how to translate) in cm, but all the other stores sell in mm. https://olx.ba/artikal/19038696/lim-u-rolni/# When you want custom bent stuff (like the metal.. things that go on the sides, or custom gutters) you make a drawing in mm (can't explain, but I'l draw one if you want).

I just said "professional" because the one I replied said it, without saying what profession. Roofing is a profession, and metal roofing stuff doing (again don't know how to translate) is also a profession.

And all the schematics I have seen for metal parts, and in cad software in general, is in mm. Tailoring is also in mm (https://tehnicki.lzmk.hr/clanak/kroj).

Maybe it is a US vs rest of the world thing ? Maybe I'm not expirienced enough to make such sweeping statements.

Again, you put thousands of mm because mixing cm and mm (and m) can lead to mistakes. I even remember cutting wrong because I heard like 2570 instead of 2507 (he's up there measuring, while I'm down cutting).

House blueprints are usually in meters. Window sizes are in cm.