The only logical means by which this could occur would be by the introduction of compulsory key escrow, and the banning of forms of encryption which do not use it. While the UK already essentially has a legal means to demand your encryption keys (and imprison you indefinitely if you don't comply), this would fall short if you have a credible reason for not having the key any more (such as using an OTR plugin for your chosen chat program).
The US tried a similar tack with Clipper in the 90s.
As we all know, terrorists with any technical chops are unlikely to be affected, given the vast amount of freely available military-grade crypto now available, and the use of boring old cold war tradecraft.
Ironically, France used to ban the use of strong cryptography but has largely liberalized it's regime since 2011.