For brush on matt varnish, I use
Winsor & Newton Galeria Matt Varnish. It's a great product, drying really matt indeed. A good trick of the trade I learned here on the LAF though, is storing the bottle upside down, so you get the 'mattest' possible product out of the bottle immediately when opening it.
For spray on matt varnish, I still use
Testors Dullcote. I was lucky to have been able to order a bunch of the small cans before more stringent shipping regulations came into effect, and I've been using mine very sparingly, as I suspect it will be nigh on impossible to have them shipped again, and there is no local (or regional for that matter) store stocking it.
For what it's worth though; I still think it's the best matt varnish out there...