Even if you HAVE got something to hide, they're still getting in. They've already blocked off all your drains before they knocked, so they're going to catch anything you try to flush. If it's just PC Plod and WPC Strongwoman, they might be attempting to blag their way in, but if they've come mobbed up, they've got a warrant. Any delay is just going to see your door feeling the warm embrace of the battering ram.
I do wonder if the reason they still use the battering ram when there are more effective techniques available is for psychological reasons. The idea that any resistance will be met with overwhelming physical force is a message that I can see the police wanting to send.