Guys, just let me make clear that plants do not require UV light to thrive. The entire UV discussion is a waste of time. Plants like it, but they don't need it. The ideal spectrum for plants is still in the visible part of the wavelengths (blue and orange and some red light).
Glass filters all light, double glass even more. A mirror reflects light dead on, the angle of entry is the angle of reflection, that kind of stuff. In grow tents and camera-thingies, they tend to use coarser material because it reflects in all directions.
One problematic thing we see annually in pot growers, is that they burn down the neighbors shed (or their own plants and house) because a mirror can cause serious concentrations of sunlight, sometimes acting like a magnifying glass. Back in the days, Archimedes devised a Death Ray with some mirrors, which didn't work that well but still people accidentally set things ablaze every summer by forgetting about it. You'd need to both counter that, and take into account that the sun doesn't stay in the same location during the day, and that shift continues to change every day. Ever seen a motorized mirror? I haven't. Too much effort to keep it going.
Coarser reflective material keeps on shining, albeit less intense, but more thorough and more evenly during the entire day. The chance to get 'hot spots' is way smaller.
Would it help? Sure, a little. But it might make your neighbors angry, since it'll be shining in their face too.
White plastics or even regular white wall paint reflects almost as much light as coarse metallic material. That's where the white color comes from..
But are you willing to change your windowsill to a somewhat building-site looking one, or do you want to keep it clean and see your trees throughout the day? That is, if we're talking window sills. I don't know about the grow area.
My enclosed indoor grow area has a nice backdrop that I enjoy, and instead of going with reflective material, I just added a few extra high intensity LED lights. With highly reflective material, I can still manually create hot spots though. So I believe white paint or scattered reflection is better.