Henry Wismayer is a writer based in London. LONDON — On the corner of a busy gyratory, a bus ride from my house in south London, sits an unremarkable yellow-brick building. Each day, thousands of commuters sweep past this corner on the way to Vauxhall Bridge. Chances are that few would give the building a second glance were it not for the diverting sign that runs between the windows of the lower f...