Other mistake: At the beginning after the cab crashes, there is no damage to the car or the post.
Other mistake: At the beginning, as Eggsy is pursued through city streets by the Golden Circle henchmen, the three pursuit vehicles are all firing rooftop mini-guns at Eggsy, in spite of the fact that they are all in single-file, one vehicle behind another. The Golden Circle vehicles would have been riddled with their own gunfire.
Other mistake: One of the robot dogs spits a shell casing after being shot in the mouth. Casings are not propelled but discarded. It definitely made the sound of a casing and not a round so it may just be an audio error.
Answer: At the time, these people only had a really short time to save the world. Merlin thought that, with the landmine, he could sacrifice himself along with many guards to save the world.