I think the main windows on most jets do block a large percentage of UV of various sorts but even 1% (or 0.1%) transmission at 30-40,000' lets in a fair bit, in absolute terms, as you are above most of the atmosphere and there's precious little between you and the surface of the sun.
When I first started longhaul, I actually got mild sunburn on a long sector. Since then (being something of a canary in terms of UV) I've used sunblock and shield myself from direct sunlight.
Nothing worse than a long flight from Europe to the west coast of the USA, sun stuck right in front of you for 11hrs... Thanks goodness for the Sunday supplements!