Hi, I use the Humbrol clear cote ( in the glass bottles not tin stuff ) all the time over Tamiya Acrylic paints and Hycote acrylic paints and have never had a problem with compatability, You must thin the varnish down with Humbrol thinners though a 2:1 match.
Never heard of it going off as you describe would more likely be that the temp is wrong and too thick maybe. You can always test a sample over the paint you use on a scrap piece to make sure though.
I personally wouldn't varnish anything with Ronseal except my front door as it goes brittle and cracks and turns yellow with age, just my opinion I guess, but would reccomend that the proper stuff for the job is used. Hope this helps :)