The problem with Ronseal polyurethane exterior varnish (and most others of the type) is that it contains UV filtering agents and therefore has a distinct yellowish cast which alters the colour of the underlying finish, the interior also affects the underlying colour but much less so unless you apply it over white. The interior variety is supposed to be proof against boiling water, weak acids, chemicals etc. etc., and many modellers swear by it. I think it can be regarded as waterproof for all practical purposes.
The quick drying acrylic stuff gives a more transparent finish but can go cloudy if immersed in water which suggests it is absorbant.