Very nice job
This makes me want to construct a UV box, it would be a similar shape to an oven with a door on the front, mirrored/reflective insides with some UV bulbs.
It will probably of course need a couple of large fans fitted as it could get quite warm from all the bulbs and will need some sort of clear shelf such as perspex or glass to place the yellowed plastic items on.
Could maybe even fit a timer and light dimmer switch (incase your away for a while you can turn down the UV intensity).
This would make a nice little project for anyone, or you might even be able to buy something like this but im not sure
UV box + retr0brite = whiteness whatever the weather