Thunder Bolt
Silly old bat
It does appear an odd conclusion by the Police, albeit under incredible political pressure which they really don't like.
A more coherent conclusion (if morally wrong) would be that since the PM was in his "home" but unable to control others being there (because of the uniqueness of Number 10), he wouldn't get fined at all.
.. but he has been for one occasion.
What a complete mess.
He didn't actually live at Number 10, because the flat at Number 11 was bigger. As you say, a complete mess.