Who'd have thought that today, of all days, it would go dark?? Anyway, it was a surprise to 4 people who becam benighted on Slapestone Edge, Grasmere. They were located using SARLOC and recovered using torches...
What ever time of year, always carry a torch. You never know when or why you may be delayed.
When it goes dark in the hills, it usually goes VERY dark! There's little ambient light unless you are lucky to be out when there is a good moon and little cloud.
It's worth carry a set of spare batteries as well. If you're lucky they'll be the same size as other devices you might be carrying such as GPS.
Modern LED torches are very reliable, and batteries last a long time, and it's well worth considering a headtorch in favour of a hand torch, leaving your hands free.