> 1. Everyone has the right to respect for his private and family life, his home and his correspondence.
> 2. There shall be no interference by a public authority with the exercise of this right except such as is in accordance with the law and is necessary in a democratic society in the interests of national security, public safety or the economic well-being of the country, for the prevention of disorder or crime, for the protection of health or morals, or for the protection of the rights and freedoms of others.
[Note the exceptions, which are (by design) wide enough to drive a bus through]
An Act of Parliament restricting end-to-end encryption for the expressed purposes of preventing crime and preventing terrorist atrocities would fairly clearly be constitutional, I think. Even if it was ruled incompatible with the ECHR, the courts have no power to overturn primary legislation - just to punt it back to Parliament with a declaration of incompatibility.
If a minister tried to do it without Parliament under the royal prerogative or secondary legislation, I think the chance of it being overturned as unlawful are somewhat higher.
I'm a political scientist, not a lawyer, mind.