One power failure in one London zone took down HSBC, Revolut and Lloyds (neowin.net)
The outage began 19 October at 23:08 PDT; Google called it a power event in a single zone. Per the Neowin headline, HSBC, Revolut, Bank of Scotland, Lloyds and Equifax were hit.
An SRE in the thread says multi-zone setups exist everywhere but zone failure is almost never rehearsed.