Cold knees means painful knees for me. Here in southern Spain, I'm fine in my usual thin tights through most of the winter, with an extra base layer for chilly days, or when I'm going into the mountains. In most of the UK, I'd probably have to wear knee warmers as well. (However, my knees have taken a battering from a few years of dancing and two accidents with cars that they took the brunt of.)
Like fossy, I'm a believer in keeping warm. Cold muscles - and tendons and ligaments - damage more easily, and that would potentially mean time off the bike.