I carried an umbrella when I walked the Thames Path and found it excellent. It was an ordinary black gents umbrella not a special piece of kit. I'm not clear what the benefit of buying one labelled a walking brolly would be, especially if there is a price premium. I didn't take a waterproof jacket and kept dry all the way. Having said that it was summer and not too windy at any point. The wind encountered in hill walking might make them more difficult to handle and up here in Shetland they would be a bit a real trial I think.
I know they've become more popular among walkers in the US but I believe that's got a lot to do with protection from the sun.