We have hit peak trouser. In the past year, designers have given us pants with waists high and low, with voluminous pleats and second-skin fits, designs that barrel and flare and taper and crop.

A new trend is taking over the streets of London right now. A frilly, wide-legged, Sienna Miller-esque trend: that of the bloomer. Think bloomers were only for Victorian ladies? Think again—they’ve ...