Ceiling at the bed, floor for the ladder and drawers, stairs on delivery day, and the mattress size the frame actually takes.
Measure from the floor to the ceiling where the top bunk will be, then subtract the frame and the mattress. A widely published UK retailer guideline is 75 cm of clear space above the mattress so a child can sit up. That figure is not a legal minimum.
On a slope, the measurement that matters is the lowest ceiling above the place the child sits and climbs. A shorty bunk is a lower, often shorter frame. If the low side is still tight, two ordinary beds are the safer layout.
A UK single mattress is 90 × 190 cm. The frame is larger, and the ladder, shelves, drawers and a desk add more. Plan the floor from the maker’s overall width and depth, then check the door still opens.
Order it flat-packed and measure the longest panel against the stair width, the landing and the turn. The assembled height is a bedroom problem, not a staircase problem.
Put the ladder on a clear piece of floor, not over a radiator and not in the door swing. Keep window blind cords out of the bed.
Most UK bunks take a 90 × 190 cm single. A small single is 75 × 190 cm. A shorty is often around 75 × 175 cm, which is shorter again. Depth is a separate limit, marked on the frame.