As it's stay-put, the assumption is that only one flat is evacuating at a time. The front doors have to be fire doors so once the evacuating people are in the staircase they have bought themselves at least 30 minutes. How, though, do they continue their evacuation down to the ground floor without a lift? It's a valid point.
It may be worth considering the contents of the CLG document,
"Guidance on the emergency use of lifts or escalators for
evacuation and fire and rescue service operations, BD 2466." This might demonstrate that it will still be possible to use lifts during a flat fire - but this may depend on the configuration of the electrical supplies. Worth investigating.
Regarding the following comment,
..... over to the fire brigade to manage evacuation.......
I'd be very careful over what you assume the fire and rescue service have the resources to do. Assuming that they will be able to manage an evacuation is probably a step too far and you should liaise directly with them before making any such assumptions. I know what you mean though, Anthony; any such evacuation, in a stay-put premises, would only be the result of some drastic shortfalls in the fire safety arrangements (e.g. lack of assumed compartmentation).
Stu