The rain in Walthamstow has a way of turning the High Street into a blurred watercolor of neon shop signs and shimmering pavement. It’s 11:30 PM on a Tuesday, and the Victoria line has just rattled its final passengers out into the cool, damp air of the station square. For the locals, this is the hour of the Minicab. While the iconic black cabs might rule the gridlocked veins of Central...