I did 10 miles tonight, 6 of those in the dark. I ran partly on and then off road (forest trail). Yeah off road wasn't a good idea, even with a good head torch, you can't really see for foot placement.
I can't see on the unlit roads either but except for adverse camber, which I know is there, no prob on that score. Some of the drivers can't drive lanes though and I do have to run defensively.
On the running as a woman alone topic. I'm going to be honest, I do get some confidence from the fact that I can outrun most people over distance. If they don't catch me within 200m they're probably not going to.
But even if I couldn't, you surely can't live your life on the what if? But I do fully understand that different places in the UK offer different dangers.