I doubt that it's to show ones manliness. When you are used to rain (I bet everyone that lived in London for more than a week is), you know when it's not strong enough to get you wet and when it's really time to take your umbrella or get a shelter. I usually carry an umbrella, but use it only when I see that otherwise I'm going to get soaked. Walking down a crowded street with an umbrella is often simply too much of a hassle.