As part of building control for a new build, I need to submit a water
efficiency statement for part G.

I've found the water efficiency calculator, but I'm having trouble
finding the flow rates for the taps and shower. The document says I
should use manufacturer's figures, but I can't find these online.

Surely any stated flow rate would depend on the pressure anyway? And I
don't know the mains water pressure (everything is on mains pressure,
with a combi).

Does anyone know if it's acceptable to measure the usage by timing how
long to fill a bucket? That would seem to be the best way to get a
true figure, but I wonder if the BCO will accept it. Any other tips
from anyone who's done one of these calculations before?