Common problem in England. Lots and lots of varnished front doors.
I checked it out years ago and one varnish manufacturer advised THIRTEEN coats of varnish was necessary to protect the door from sun rays!! May be overkill but it does indicate the extent of the problem.
However, I built and lived on a sailboat in California and varnished all my wood with "Spar/Marine" varnish and I had no problems - but that was only over 3 years. I'm sure I would have had to recoat shortly after.
Whether you use marine varnish or an exterior Polyurethane, be sure to buy the one that states "UV Protection". AND be prepared to recoat every few years.
Bottom line - both your friends were in fact correct. The best protection is a solid stain/paint but you can maintain a natural wood finish at a little extra cost and effort.
Hope that helps and good luck. Alan