I can't tell whether this is hubris, technical naïveté or just an attempt to garner votes. Possibly all three. But let's assume they are proposing a ban on communications methods that cannot be lawfully intercepted (that's certainly how Cameron words it).

Firstly, we know it's technically infeasible, secondly it would have a serious chilling effect on UK Internet companies and the digital economy, and thirdly it would actually take very popular apps and tools from young voters hands. So does he really think people will be panicked enough by the threat of terrorism to vote for it?