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.
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.