Other mistake: When they are in the trench the other sub fires two torpedoes at them. The captain calls for right full rudder. They then show an underwater show of the sub turning left. After two more torpedoes are fired the captain orders left full rudder and they show the same underwater shot of the sub turning left.
Other mistake: When the parent monster is just about to enter Piccadilly square, for a brief moment a view of a bus is shown. The writing on the bus is reversed.