The air is bright with spring sunshine in central London. We’re here for a midterm break holiday. We take a cab across Waterloo Bridge to the Imperial War Museum. We’ve been traversing the city on tubes and buses, loving that feeling of living like the locals, swiping our bank cards on public transport – no fiddly tickets or Leap card-equivalents needed here – wishing we could do the same at home. The pricier cab is a treat. Buses and tubes are brilliant, but in a glass-roofed black cab, you can cross the Thames, Big Ben visible above your head, the...