SK,
Not to put words in Mr. Dean's mouth but I think his main complaint was about a consistency of perspective. While it's perfectly legal to change perspective in a story it's often very confusing to the reader.
Most of his problems seemed to be with a consistent voice that fits the character. The best way to develop a feel for that is to listen carefully to real people in your everyday life.
You should also be aware that few people follow the rules of grammar when they speak. If you put it inside quotes you can get away with a number of grammatical sinsJust be sure they fit the character that's speaking. For instance your pompous (bemused?) Brit would enunciate clearly, chose his words carefully and probably have a dry wit. He’d most likely be enthralled with his own clever plays on words that were meant to go over the head of his interrogator. That kind of smug jibe could be exactly the kind of thing that would cause the policeman to snap.
The police would be more likely to use contractions colloquialisms and slang. You have to be careful not to overdo that but a sprinkling will add some depth and believability to your characters.
I'd have given it an 8 and encouraged you to post more.
Mad Lews