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: Harry probably believed Whiskey told the Golden Circle where they were since none of them were followed. Then there is also that Whiskey shoved the antidote out of Eggsy's hand he started to suspect he did it on purpose.