Yep, she's fucked up. But it could so easily have been nipped in the bud. She could have just said, "hold on a second, my mind's gone blank, let me have a little think", paused, took a drink or whatever and gathered her thoughts. The point that is ignored is that the policy is quite sound (if you want more police, which I don't), the numbers were correct in the policy and the costing met. That is the only thing that matters. Not whether or not some random wifie has remembered her lines correctly. It's not fucking dragon's den. Personality politics once again.
The problem they have is that there wasn't a clamour from other labour politicians to defend her. Which says more about the party than it does her. The Tories would have dealt with it quickly and aggressively. Although it wouldn't have made the news if that incompetent fuckwit May had said it.
They have no option to ditch her though.