I’ve been to the some of the most trailer parky of methtopias that America has to offer in our great and varied nation. I’ve seen things done with peanut butter that would have caused god to nail himself to a cross in order to escape (these people make peanut butter sandwiches to dip in soup. ANY soup. (What came first? The chicken and dumpling or the peanut butter dipper? Come back in a few thousands years for a third try Jésus, these people walk amongst peanut butter beef stew and sin) ). I can say I’ve never seen anyone in any echelon of society apply butter to a peanut butter sandwich. We’ve committed many atrocities, but this one our hands are clean on.
Toast melts peanut butter, it’s already going to soften it similar to butter. When making a non toasted peanut butter sandwich you don’t want it to soften, so no need for butter. I suspect this is something localized to the former empire, similar to the butter sandwiches they consume across the pond. Buttered toast as an open face thing is definitely a thing here, but turning toast into a sandwich and using butter as the main filling is definitely a British thing.