More and more UV resistant epoxies enter the market.

http://www.mertons.com/Epoxy/epoxy_resins/sb.html

A week ago someone had an epoxy application question and he was using another UV resistant epoxy brand. How UV resistant or "proof" they are, who knows, but you can contact the manufacturer. However nothing is gained by not varnishing in your case. You would have to sand any repairs anyhow and to cut through the varnish will be seconds with a ROS.

Sail boat races are won sometimes by fractions of a second. The hull prep makes for good reading and if you invest a million into the boat, you might as well tweak if for all it's worth. For a kayak, you will gain nothing measurable. Smooth is better than rough but that's about it.