That twelve figure looked high to me also, altho widely quoted.
Here is a pretty good cite IMHO-
https://www.postalmuseum.org/blog/health-pandemics-and-the-victorian-post-office/
With up to eight deliveries a day in London, the postman was someone with whom members of the public, particularly the middle classes, came into contact on a regular basis. The question of whether sickness can be contracted as a result of contact with these vital ‘key’ workers of nineteenth-century Britain was, therefore, a pressing one for the Post Office.
And of course the Square Mile was different than Metro London.
In other parts of London that makes sense. But if you were in the Square Mile, things moved very fast indeed during business hours. Maybe even hourly, sure at the height of the Postal service.
We have covered this before-