Mike,
Polyester resin requires no UV protection. It isn't a great transparent finishing product like epoxy - it tends to be a little bit 'milky' - if you're going to the effort of building a woody with nice wood you want to look at, epoxy and then a UV-blocking varnish are the way to go.
I'm a little nervous of the generic 'spar varnish' description, particularly if you're not looking to recoat the varnish every year or so - the quality of the varnish used over epoxy is probably more important than the quality of the epoxy. I certainly wouldn't trust any single-component varnish in this use, if I wanted it to last. A two-pack polyurethane varnish intended for marine use would be my choice - very expensive, but you don't regret the cost when you don't have to sand and re-varnish!
Andrew