Everyone has assumed that when Cameron says that he wants to "ensure that terrorists do not have a safe space in which to communicate", that means that he wants to mandate encryption backdoors.
I think that assumption is incorrect. The guys at Number 10 are well aware that (a) they can't ban math, and (b) the UK tech sector would suffer if they legislated to mandate backdoors in all encryption.
I suspect that what Cameron actual means is that he wants to put in place a legal mechanism to authorise GCHQ (presumably by issuing a warrant) to hack into suspect's laptop or smartphone in order to gain access to the content of whatever messages or communications they may be exchanging with any co-conspirators.