If the latest information is true (http://www.londonist.com/archives/2005/08/charles_de_mene.php) then it seems to me the police who killed Charles De Menezes should be charged with nothing less than premeditated murder. It appears that what happened is a man ran to catch a subway train in the usual way, got onto a car, sat down, found himself startled by police who then subdued him. Having subdued De Menezes, the police used that opportunity to shoot him in the head. The reason for this shooting appears likely to have been that De Menezes ran to catch his train, and the police had already decided to shoot him if he 'ran'.
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.)