Bara brith toast gives up the second slice on purpose, and the reason is the crumb. Bara brith is a dense Welsh fruit loaf, the name meaning speckled bread, heavy with dried fruit soaked before baking and close-textured rather than airy. A loaf that solid does not want to be the wrapper of a sandwich; it wants to be the base of an open face. Toasted and buttered as a single slice, eaten with a knife or in the hand over a plate, it is the form the bread is built for, because a second slice on top of something this dense would only be more weight with nothing gained.
The craft is heat against a fruited, sugar-laden crumb. The dried fruit carries sugar that catches and scorches faster than plain bread, so the toasting is taken to firm and coloured rather than dark, watching the fruit at the surface rather than the crust. The slice is cut thick because a thin one of a crumb this close is brittle and a thick one toasts to crisp at the faces while staying moist and dense inside, which is the contrast the whole thing rests on. Butter is the only addition that matters and it goes on while the toast is hot so it melts into the open fruited crumb rather than sitting on top, salted butter against the sweet loaf doing the same counter-job the salt does throughout the British sweet shelf. There is no filling and no second slice because the bread is already the event.
The variations stay open-faced and change only what sits on the butter. A spread of salted Welsh butter alone is the standard. A slice of a firm cheese such as Caerphilly laid on the warm toast turns the sweet-savoury balance toward a Welsh tea-table plate. A thin scrape of marmalade or honey pushes the sweetness further. Each deserves its own article rather than being crowded in here.