Earlier quoted context omitted.
If encrypted communications are illegal, then anyone using them is an automatic target for investigation and the encryption itself may be probable cause to open up their lives and property. It's considerably more difficult to conceal the fact of encryption on the wire than (say) a gun in your home.
Encryption is not illegal, but even if it were then everyone should still use it, because not using it reveals much more information. In a society were encryption is illegal or even seriously treated as "probable cause", then the authorities are likely to pick out anything in your day-to-day activities to use against you in an offensive legal process. So "not using encryption" is a fool's defense that makes literally…
It needn't be the security services, or even the police; cases can be brought by the taxman. This has been the case for the last 10 years or so, the new law will extend these powers.
https://wiki.openrightsgroup.org/wiki/Regulation_of_Investig...