London Has Fallen is a mediocre, yet acceptable sequel to the 2013 movie, Olympus has Fallen. It follows the same formula (actually Die Hard's formula) beat by beat and delivers an entertaining experience (that would be expected by the fans of its predecessor).
Gerard Butler has not had much luck in the past few years (Gods of Egypt, and a slew of romantic comedies); the role of Mike Banning in Olympus and London is his saving grace. I wish James Bond producers (Michael G. Wilson and Barbara Broccoli) would see these movies and take them as Butler's audition for James Bond - more on this in another post.
The director (Babak Najafi) does a respectable job in his first major Hollywood blockbuster. The effects are decent and the pacing is good. The dialogue is really cheesy at certain points - particularly the scenes at the Situation Room in White House - which is scriptwriters' faults, not the actors or director.
This is a short movie, and there isn't much to be said about it. Just to reiterate my earlier points: it is Die Hard in London, the story and dialogue do not make much sense, but the action is good. With low expectations, one could enjoy this as a mindless popcorn movie. As a movie in the said category, I give it 7/10.