On a dull, gloomy, day, your photographic options can be limited, but you can make best use of days like this by choosing a subject that does not need strong, bright light. Waterfalls make an ideal subject on such days.
The place to head for is the little hamlet of Ystradfellte, in South Wales, sandwiched between the rivers Afon Mellte and the Nedd Fechan, both of which have photogenic waterfalls within a short walk of one another.
This is the unnamed waterfall just below the carpark at Pont Melin-fach on the Nedd Fechan. Days of heavy rain have left the rivers on full spate (and the rocks very slippery indeed!).
This was comprised of two images, stitched together (rather painlessly) in Lightroom CC.
Photography stuff: 12-24mm lens at 24mm, 4 seconds at f/13, tripod, remote release, polarising filter.