What sauce for a XYZ butty? Is certainly a difficult question.
It really needs a Venn diagram or something.
My personal rules are:
Bacon butty = ketchup
Bacon and egg butty = HP sauce
Sausage butty = ketchup or HP sauce depending on mood
Sausage and egg butty = HP sauce
Full on BSE = HP sauce
There is a definite weighting towards HP sauce.
Inclusion of egg requires HP sauce. Sausage trends towards HP sauce, but I would not be angry if presented with a sausage butty with ketchup.
Not that I have any of the above often. Usually a once a month treat, which is why it’s important to choose carefully.
I tried ketchup on a BSE a while back and came away from it feeling a bit cheated.