Character mistake: Galahad claims that classifies Brogues as being 'mutually exclusive' from Oxfords. Broguing is any decorative hole punching on shoes and can be found on any shoe style.
Character mistake: When giving Eggsy a tour, Galahad tells him that Oxfords are any formal shoe with open lacing. This is incorrect as Oxfords have closed lacing.
Character mistake: Anyone who knows anything about firearms would never handle a firearm with their finger on the trigger unless ready to shoot. In the training scene with the dog Eggsy is running with his finger on the trigger the whole time. Merlin also does it in the end scene at Valentine's secret lair inside the jet.
Answer: Details aren't given, but it seems that Dean is involved in local crime along with the gang that hangs around with him. Even if Harry is bluffing, and doesn't have specific proof, it is clear that he is abusive and he has the recording of him threatening Eggsy.