What's more, Kristy tells me that the only reason they were following Charles De Menezes in the first place is that the guy who was taking pictures on the block where De Menezes lived was taking a bathroom break when De Menezes left the building, so he called ahead to get other cops to check De Menezes out. (P.S. The article I read says that. I was not completely with it when I wrote the previous, I guess. :) )
The reason for the lies and cover-up is apparent -- the police understood that they had just murdered a man in cold blood, and in fact had premeditated that murder.
(Note I am no authority on British law, nor do I care to be.)