If Hovis is a northern brand, and not something you'd ordinarily get in Dorset or other West Country parts, then yes, that would explain it. (I'd never heard of Hovis till reading this very post.) The Wikipedia article for Hovis suggests that this advert is what made it nationally famous, so perhaps the delusion was not the result of a pre-existing association, though.

That my tourist impression is to associate both brass band arrangements of mournful classical music and really steep cobblestone streets with Yorkshire is probably not very significant. I've seen hilly town streets in southern England, but not as steep or as frequently as in Yorkshire, and the fact that this street has its own tourist website does suggest it's a bit unusual.

As for Ridley Scott, is he likely to have been well-known as a director before Alien (1979)?