If you need to empty the cistern, just stop the ball cock from dropping and the valve won't open. Flush the water out, job's a good'un. I don't see why you'd need to stop the supply anywhere else.
Am hoping no one comes back with a reason this wouldn't work... Potential so obvious it's genius is potential.
@CYOA started
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If you need to empty the cistern, just stop the ball cock from dropping and the valve won't open. Flush the water out, job's a good'un. I don't see why you'd need to stop the supply anywhere else.