By banning real encryption, the police can arrest anyone using chat apps they can't control. Them not being able to spy on you automatically makes you guilty.
Also something something children something terrorism something something safety, whatever makes the uninformed public feel like this is something that'll benefit them.
The criminals will just use something that hides in plain sight.
I think it was Encrochat devices that booted into a fairly vanilla looking OS, and there was another platform where it was hidden in something like a calculator app.
It isn't about criminals. It is about population control. Keeping the plebs in their place, lest they vote unsavoury opposition parties, or get ideas about real democratic participation.
Also something something children something terrorism something something safety, whatever makes the uninformed public feel like this is something that'll benefit them.