The British BLT is decided before anyone tastes it, in the few minutes between toasting the bread and closing the sandwich. Bacon, lettuce, and tomato on toast is a simple list, but it is structurally precarious in a way the list does not show: hot bacon and cold wet tomato meet on a slice of toast that begins softening the moment the tomato touches it. Get the timing wrong and a sandwich that should be crisp and sharp arrives limp and pink-bottomed. The defining fact of the BLT is not its three letters but the race between the toast going soft and the sandwich being eaten.
The craft is moisture management, and the tomato is the offender. A tomato slice laid straight onto toast weeps juice and seed gel into the bread within minutes, so the fix is structural: salt and drain the slices, or hold them off the crumb with a mayonnaise layer that waterproofs the toast and seasons the sandwich at once. The bacon is the load-bearing flavour and is cooked properly crisp, both because it tastes right and because a crisp rasher resists sogging where a soft one surrenders to it. The lettuce is there for cold crunch and is kept dry. The toast is taken a shade further than for a soft sandwich so it has structure to spend, and the whole thing is built and eaten quickly, because every BLT is on a clock and the only winning move is to be fast.
The BLT is the parent format and its codified mutations change one variable each. Adding avocado makes a BLAT, trading some of the crisp tension for richness; chicken or turkey turns it toward a club; the third slice of toast that braces a tall stack is the club's whole answer to the BLT's instability. Those deserve their own articles rather than being crowded in here.