The British club sandwich is a three-decker, and the third slice is the whole engineering argument. Two layers of filling, chicken or turkey with mayonnaise in one and bacon, lettuce and tomato in the other, are separated by a slice of toast through the middle, the lot pinned with cocktail sticks and cut into four triangles. In the British café and pub register where it lives, this is a plated, knife-optional order rather than a deli counter one: it arrives on a board with a garnish and a few crisps or chips, eaten sitting down. The defining fact of the build is that the middle slice is structural, not generous. It braces a tall, soft, wet stack so it can be cut into quarters and lifted without the layers sliding apart, and it keeps the wet BLT half from soaking into the bird-and-mayonnaise half.
The craft is stacking, bracing and moisture control across two fillings at once. The toast has to be properly crisp, because soft bread cannot carry two filled tiers and survive a knife through it, and it is the toast going limp under wet tomato that kills a club faster than anything. The tomato is therefore the managed enemy as it is everywhere: salted or drained and kept off the bread by the mayonnaise layer, which doubles as the waterproofing and the seasoning. The chicken or turkey is sliced or bound thin so the tier compresses rather than splays; the bacon is the load-bearing flavour, cooked firm. The cocktail sticks are not decoration, they are the fastening that lets the structure be quartered, and a club assembled and then cut without them collapses back into a tall, unstable BLT. The British café version tends to back bacon over streaky and a soft white or granary toast over the American white, which shifts the register without changing the architecture.
The variations stay inside the three-decker frame. The plain BLT is the club with its middle slice and second tier removed, the parent the club was built to stabilise. A BLAT adds avocado and trades crisp tension for richness; an egg mayonnaise tier swapped for the bird is the common British café substitution; a club made with fish fingers or prawn instead of chicken is the seaside reading. Each deserves its own article rather than being crowded in here.