It has nothing to do with the brushes: I just mentioned that as one of the wonders of modern varnishes.
I don't know if they are all like it these days, but at least some of the Ronseals I've used do wash out pretty well with water: they look kind of cloudy in the tin but dry clear - and I generally go for the 'diamond hard' and find it excellent: and a little goes a long way so it is not as expensive as it seems. They do tend to drip, so the finer the layer you apply the better, and that is where these thin sectioned artists brushes come into their own. (It is a year or so since I last did any varnishing - though there is still plenty needs doing! - so I didn't stick my neck out here to say whether I thinned my last lot with white spirit or water.).
Incidentally I've also used the same brushes with very thinned gloss paint to get a smooth finish on the fiddly bits of window frames too..
S