Matthew's character "Denton" tells his men to line up 20 yards South by Southwest; being a Texan he is referencing Austin's annual music festival, South by Southwest (SXSW).
It was filmed in Ireland, in the Wicklow Mountains. They were only allowed to film there under the condition that they clean up entirely after themselves, without damaging the landscape.
In the opening scenes, the small boy appears to be walking in London around the new Underground building site, but clearly in the background two red and white chimneys are visible. These chimneys are located in Ringsend, Dublin (where this scene was filmed), and not London.
Van Zan tells Quinn the explosive crossbow bolts are magnesium-tipped C4. Assuming the magnesium is being lit from impact or from the dragon's breath, the C4 would not detonate; C4 detonates only with combined heat and pressure usually only obtained from detonating a blasting cap embedded in the C4. Unless the magnesium tip was purposed as the ignition source of a blasting cap that wasn't mentioned in the arrow head the C4 would not have detonated.
Van Zan mentions that he was able to kill a dragon during "magic hour" -- what was he talking about?
A
Van Zan was talking about a specific time of day near sunset called "magic hour" or "golden hour", when the light from the sun is softer. Van Zan had deduced that the dragon he killed (and the rest of its species) have impaired vision at that time of day. We see evidence during the final battle with the male dragon in London: while it hunts Quinn, Van Zan & Alex, we see things from it's perspective, which is distorted & somewhat blurred, making it harder for the dragon to see them.