You see, at the time, the British thought that umbrellas were not only for sissies, but for French people. Umbrellas had existed in some form for thousands of years, but they didn't really catch on until France made them more convenient in the early 18th century. So to England, umbrellas became a symbol of Frenchness, like baguettes or going to war with England.
At best, people laughed at Hanway for being some sort of fashion-obsessed dandy. At worst, they tried to run him over with stagecoaches. Coach drivers were particularly incensed by this new technology because to them, rain meant money. If people realized there was a much easier way to get out of the weather, they'd lose business. When one of them tried to "solve" the problem by charging at Hanway with a hansom cab, his response was to demonstrate another use for his umbrella by "giving the man a good thrashing." England slowly warmed up to the umbrella, perhaps fearing they'd give France an offensive advantage in their next war.