This has nothing to do with stopping terrorists. I'm pretty sure Cameron is not a stupid person and has consulted with experts on the matter, and there's no way anybody with any kind of clue is going to think it's possible to stop terrorists from using strong crypto.

No, this sounds like playing the Terrorist wild card to ensure the common people don't get crypto. Cameron probably fears that your average (criminal) Joe, with the help of tech companies, transparently gets to use crypto. Crypto is getting to easy to use.