Since this involves a time before written records, this may not be the best subreddit for the question. At the same time, since it is involves a time before written records, it is largely impossible to answer your question. I can say that with my experience involving winter in Britain and Ireland, the average day was indeed cloudy, but the sun more often than not appeared on the horizon as it rose, shined for a few minutes, and then rose into the bank of clouds. Stonehenge only "works" at the solstices at the moments of sunrise and sunset; not later when the sun is behind the clouds.