I still dont think thats the Enterprise crashing, everything about the silhouette suggests its not the Enterprise + if you freeze frame just before it hits the skyscraper the hull does not seem the same, its more curved.
When we first see the Ent falling into Earth(?) from orbit the ship still has power but when we see it in the atmosphere it has no power because the running lights have switched off. This may explain the gravity issues we have seen in the trailer? The power is off and the anti-grav has failed The Undiscovered Country style only this time they are in a planets atmosphere and experiencing g-forces!
Also Union Jack upside down was flown on any ships in distress.