I'm very much northern, mum from Liverpool, dad from Southport, live in Northumberland. I dunk ginger nuts but only in tea. I wouldn't want to spoil a cup of coffee. We were brought up with a slightly ambivalent attitude towards dunking. My mum was OK with it because my grandparents did it. My dad secretly liked doing it but, coming from the rarified air of Southport, his mother disapproved although she was from a very lowly background indeed, by all accounts (nuff sed!) Anyway, I always thought my grandparents did it because it softened the biscuit and made it easier to eat. They always took their ill fitting false teeth out at every opportunity!
I'd never dunk cake! Surely great chunks would fall into your cup?