Banana and honey is a sweet sandwich whose defining problem is that it puts two wet things against bread at once. Banana weeps as it is cut and honey is already liquid, so the build, sliced or mashed banana and a drizzle of honey on buttered white bread, carries more moisture toward the crumb than almost any other sweet sandwich on the British shelf. Everything about how it is made is a response to that. It is a sandwich that has to be eaten promptly and built with the bread defended, or it slumps.
The craft is moisture management and the bridge between two soft sweetnesses. Honey runs, so it is applied in a restrained drizzle rather than a flood, and it is laid onto the banana rather than straight onto the bread so the fruit catches it before it can soak through. Butter to the edges does the structural work, waterproofing the crumb against both the honey and the banana's own bleed and, being salted, supplying the one savoury note that stops the doubled sweetness reading as cloying. The two flavours are close, soft floral sugar over soft fruit sugar, so the value of the sandwich rests on freshness and on the salt and the bread doing the contrast the filling cannot do for itself. The bread is plain and soft so it yields rather than competing with a filling that has no structure of its own.
The variations mostly add the body and texture the plain version lacks. Peanut butter on the opposite slice is the common one, supplying salt, fat, and a bind that also slows the banana browning. A scatter of oats or seeds into the honey adds the crunch a filling of fruit and syrup cannot bring on its own. Honey alone on buttered bread, without the banana, is the stripped-back relative. Each deserves its own article rather than being crowded in here.