Lord Spandex Masher,
I have never worked at a company that has required the cabin crew to seek my permission to evacuate. Nor have they been required to let us know they were going either.
They are trained to check outside. Hopefully that's what they'll do!
So it's perfectly possible that you are waiting at the holding point and for some reason cabin crew starts an evacuation without informing the cockpit?
Ok, no problem, AC is standing still and outside conditions are good.
Meanwhile, you get permission to line up and you start moving when the first passengers start to jump... I don't think it will be a pretty sight.
My company's policy:
Captain decides for evacuation after an inital call.
After this call, checklist for evacuation is being completed. (My main concern are the engines)
If evacuation is required, the captain will make the evacuation call. If it's not required he will cancel the initial call.
Exception:
If there was no initial call from the flightdeck e.g. after an runway excursion (which indicates pilot incapacitation), CCM are allowed to evacuate.
If for some reason CCM feels the need to evacuate, inform the cockpit. E.g. a fire which the FCM don't know about.
Offcourse, the example that IGh has given, where the captain delayed evacuation, was poor decision making from the captain.