Well he's a cop in the "magic" division of the London Police. And they like to keep the "magic" hidden from the public, because if the public thought magic was real, then the police would have more work to do, and even worse, more paperwork to fill out.Never read the books, but these kind of things also always bug me. I suppose from your phrasing, that part of is cover is not being able to cast spells?
So doing magic in broad daylight where he can be seen seemed to go against the world building as established so far...

