Agree that cabin crew do not need to get permission for an evacuation. I'm sure that whoever initiated the evacuation had a very good reason for doing so. There will be an AAIB report into the incident which will be made available when it is completed.
addition: The report is out and a link to the pdf is available at
Air Accidents Investigation: Boeing 757-204, G-BYAT (having trouble opening pdf but that's probably my computer and not the AAIB's fault).