No its not a good safety feature as Paul has said. You could be driving at dusk and the instrument lights come on and you assume the main lights are on when they aren't.
As others have said, I find it to be a great safety feature. Your UK model might be set up a bit differently, but I noticed this unusual approach to instrument lighting the first few days I had my car.
In broad daylight, with the headlights OFF, the instruments light up nice and bright for readability.
Drive into a low-light area, and the instrument lights dim so that they become hard to read, and the driver is prompted to switch ON the headlights to restore full instrument lighting. The sensor is sensitive enough that even the tree canopy on my block produces enough shade to dim the instrument lights during the day. By the time dusk comes around, it's dim enough outside that the instruments are also dim, and I am reminded to turn on my headlights. To see this in action, drive into a parking garage or under a large viaduct and watch the instrument lights dim completely over a few seconds, leaving only the needles glowing.
The advantage of this system is crisp, bright instrument lighting in the daytime without the risk of forgetting to turn on your headlights. I see this all the time; people driving around the well-lit streets and highways of Chicago with no visual cues that their headlights are OFF because their instruments are still brightly lit.
One thing, though: if the sensor hardware is there (and it seems to accurately judge light conditions), why not just have automatic headlights as well? Despite the usefulness of the system, it seems like a pretty complicated way to simply remind the driver to turn the headlights on.