Golden - available in UK via Jackson’s at least..has replaced fluid matte, heavy body matte acrylic lines with So Flat Matte..so flat because it’s got levelling agents..matte because..well doh..anyway, my head hurts and is too foggy from treatments to actually paint anything these days, but as some sort of diversion I ordered a few and compared to AK “dense tube” acrylic- essentially very similar to Scalecolor Artist tube..except no pigment info is given whilst Scalecolor does..I have to say the Golden is very good stuff...bit sensitive to too much water and best thinned with mat medium..dead matte...maybe and it’s a maybe my brain can’t sort right now, but maybe AK is smoother..and definitely AK has white added to most of the colours which is pretty much the norm for hobby paints, especially to achieve matte and opaque effects with otherwise transparent pigments..
not trying to review, unless someone wants one..I asked and Golden doesn’t recommend airbrushing - they do dry fast and might benefit from drop of retarder...the range isn’t huge and has gaps in some of the more transparent cool reds- alizarin, magenta..and when I asked they say they couldn’t get the desired matte and opaque results..some of the transparent pigments (pthalo green) have white, some are mixed colours ..but colour charts etc are on the site..the video stuff they have up tends to the craft/canvas painter ..but I’d say the line is worth a shot depending on your local re;active prices, inclination to mix vs convenience sets..am thinking they would work well as base layer for subsequent finishing with oils..

Btw, my local pricing in C$ for Golden so flat varies per pigment 2 oz /59 ml and 4 oz tubs..they are hard to open without burping a blob, but don’t need much if any shaking...from $12 to 25 roughly vs $9 for 20ml tube of AK ...and the stuff is way denser than any dropper bottle product I’ve seen..or P3/Citadel...

I have just tried these paints and they are simply beautiful for freehand painting with only minimal dilution required.
They remind me a lot of PVA (polyvinyl acetate) paints in the way they behave.
They are perfectly matte, flat leveling and heavily pigmented. Not only that, but they do not dry on the tip of my small number 0 and 1 brushes. Color shift is minimal.
So for one stroke freehand painters like me, these are a must have, especially the opaque monopigmented colors.