Hi Ian. As you probably know, I'm not particularly a fan of acrylic paints, probably as I've been used to Humbrol enamels for over 46 years! However, I have had occasssion to use Tamiya and Humbrol acrylics, albeit in small doses. The Humbrol product seemed OK and, if anything, worked better than the Tamiya when brushed. The big disadvantage I see is the small volume they are supplied in. As far as airbrushing is concerned, I have only used tamiya acrylic and found them 'different', but easy to spray. The problem was, the paint dries VERY quickly, clogging the airbrush and, if brush painted, I was unable to use the paint itself to obtain variouis effects I can achieve with enamels. I'm still undecided, as acrylics do have some merits, but i don't think enough to sway me from enamels, even though the formulation of the latter has changed quite a lot over recent years.
Humbrol do a clear matt acrylic varnish, which I use, not sure about gloss yellowing. But, if you want acrylic gloss varnish, buy Johnsons Klear -- 250ml for £2.60 approx, as opposed to £1.25 for 12ml acrylic, Humbrol or Tamiya!!!
You can also add matt acrylic varnish to Klear, and use it as a thinner. Not sure if Humbrol do a thinner for their acrylic, but Tamiya do, and it's VERY expensive for what you get! Water, preferably but not essential, distilled, works fine.