I saw this on the BBC the other day, it made me go look up how many bus fires there are.

242 over the past couple of years (although it seems these are mostly diesel or hybrid rather than battery), there’s reasonable detail in the summaries further down the page by cause: https://www.gov.uk/government/publications/investigation-int...

I couldn’t find a way to estimate fires per miles driven.

It also doesn’t include all bus fires, only those reported to the DVSA. London Fire Brigade reports a bunch more in their publication.