On Tuesday afternoon in the City of London, a helicopter circled repeatedly overhead. Perhaps it was a news chopper, waiting to capture scenes of traumatised traders tumbling out of their offices in shock and despair.

The crew would have gone away empty-handed, but the quiet streets belied the turmoil within. Markets in Britain – the world’s fifth-largest economy (or sixth, depending on who is measuring it) – were on fire and gyrating in ways even grizzled veterans had never seen before