Well, you see, it's like this: First, they had to fill out the paperwork to request their weapons from storage. Then that paperwork had to go up the chain of command at least three levels. Then they all had to stand in formation and ask "Mother, may I?" three times. Finally they all bundled into their police van and started off.
"Whoops, we forgot the bullets!" So they rebundled back to Woolwich Crown Court and started the whole paper process over again, this time using the bullet paperwork.
That they made it in under 30 minutes is a miracle and should be applauded.
They probably had to consult their "How to Shoot a Gun" manual.