However the emergency exits leading to the roof were locked, a real tragedy
After the 1993 WTC bombing, in which a number of people were lifted off the roof, it was decided that helicopter rescue was not appropriate for large incidents, hence the reason why the doors were locked eight years later. I don't have the source material in front of me so I can't check why that decision was made.
Unless they could have gotten military choppers there quickly those poor souls on the upper floors were doomed as soon as they closed the airspace
That's not actually true - in one of the towers (South Tower I think) one of the staircases remained usable until the building fell, but in the confusion, only a very few people above the impact zone found it and made their escape. The rest stayed put or headed to the roof.
For the Grenfell Tower fire, the combination of darkness and smoke would make it very tricky to carry out a helicopter rescue. Some years back, a Wattisham-based Sea King did successfully winch people from the roof of a (much smaller) block of flats which had caught fire, but that was in daylight and the fire was contained on one floor. The RAF News carried the story on the front page, under the headline 'Towering Inferno', complete with picture of Sea King hovering above the roof with smoke-stained windows a few floors down - but hardly comparable to the recent tragedy.