Errors in geography: It is impossible to "stay tight to the cliffs" all the way from Northumbria and London: East Anglia and the Thames basin are flat.
Continuity: Quinn, Van Zan, and Alex Jensen climb up a ladder, with Quinn at the rear, yet he is the first to step off when they reach the top.
Continuity: The wound on Quinn's chin after the fight with Van Zan changes position in the following scene.
Revealing mistakes: The length of Van Zan's beard changes frequently.
Continuity: The harpoon gun that Van Zan uses to bring down the dragon attacking the castle has a bent targeting reticle when we see it from the point-of-view shot as it is fired. However, when we see the gun from the reverse angle, as Van Zan mounts it, the same targeting reticle is a perfect circle.
Continuity: The type of helicopter carrying the main characters appears to vary from shot to shot during the journey from Northumberland to London, sometimes appearing to be an Agusta A109 with a conventional tail rotor, but in other shots it appears to be an Aerospatiale Dauphine with an enclosed "fenestron" tail rotor.
Continuity: As Alex hovers her helicopter and bids farewell to Quinn, Van Zan's military convoy is seen in the background moving back towards the compound rather than towards London.
Errors in geography: Part of the film is set in treacherous mountain terrain, allegedly in Norfolk - famously one of the flattest counties in England, with only one hill rising over 300 feet above sea level.
Errors made by characters (possibly deliberate errors by the filmmakers): When the female American helicopter pilot jumps off the landing pad, she is introduced as a member of the US Calvary instead of US Cavalry.