Pen: Cornwall, yes! The Hovis advertisement used Gold Hill in Shaftesbury, Dorset, but I don't know how steep it was.
On San Francisco, I found this:
https://priceonomics.com/the-steepest-streets-in-/ containing:
"With an astonishing 41% grade, Bradford Street, in the hilly Bernal Heights neighborhood, is the city’s steepest (at least of those surveyed). Admittedly, this stretch is quite short: the majority of Bradford Street climbs steadily at about a 24% grade before exploding into a 30-foot stretch of 41% paved road."
Maybe the really steep part wasn't long enough? It isn't clear whether it is intended for vehicles, either, despite the mention of what would happen to them on it.