Firstly, there is no "everyone is innocent" option for a road collision. It happened for some reason. At the moment, in theory, everyone and everything is considered equally guilty; in practice, the dead rarely defend themselves.
Secondly, universal "innocent until proven guilty" was lost when the Regulation of Investigatory Powers Act was passed, if not before.
Yep, that's exactly what can happen under RIPA: "In proceedings against any person for an offence under this section, if it is shown that that person was in possession of a key to any protected information at any time before the time of the giving of the section 49 notice [to provide the key], that person shall be taken for the purposes of those proceedings to have continued to be in possession of that key at all subsequent times, unless it is shown that the key was not in his possession after the giving of the notice and before the time by which he was required to disclose it"
http://www.legislation.gov.uk/ukpga/2000/23/section/53 - in other words, you have to prove that you don't have the key any more. How do you prove you don't have something? If you show absence of the key in any one place, they'll just suggest you had it stashed somewhere else - you're guilty until proven innocent and what's more, you can't really prove you're innocent.