Ten boroughs, very different buildings
Greater Manchester is not one estate but ten, and the differences matter more than they might appear. Solid Victorian masonry in parts of Manchester and Salford behaves quite differently from the lightweight partitions of a recent academy build in Trafford or Wigan.
Solid construction blocks sound effectively, which usually means more devices to achieve the same coverage. Open-plan modern builds need fewer devices but raise different questions about how you secure spaces that were designed to flow into one another.
This is the practical reason we survey each site rather than pricing from floor area. Two schools of identical size in different boroughs can need noticeably different specifications, and neither figure would be wrong.