IIUC, HA setting only failover across *zones in the same region*. If the whole region is down, HA won’t be helpful. In this case, the London data center is the region.
The region wasn't down though. Only one zone was down?
From earlier in the incident history:
> Cloud SQL:
> Impact/Diagnosis: Non-HA instances backed by europe-west2-a are hard-down in europe-west2-a. HA instances that were in europe-west2-a when the incident started, are down with stuck failovers.
From earlier in the incident history:
> Cloud SQL:
> Impact/Diagnosis: Non-HA instances backed by europe-west2-a are hard-down in europe-west2-a. HA instances that were in europe-west2-a when the incident started, are down with stuck failovers.