The government already has the power to force technology firms to act as it wants over end-to-end encryption, but is avoiding using existing legislation as it would force it into a battle it would eventually lose, security experts have said.

The Investigatory Powers Act, made law in late 2016, allows the government to compel communications providers to remove “electronic protection applied … to any communications or data”.
https://www.theguardian.com/technology/2017/mar/29/uk-government-encryption-whatsapp-investigatory-powers-act

Also see:

Call for encryption ban pits Rudd against industry and colleagues
Amber Rudd’s call for “no hiding place for terrorists” on the web echoes and revives David Cameron’s 2015 proposal to ban end-to-end encryption on services such as WhatsApp.
https://www.theguardian.com/technology/2017/mar/26/amber-rudd-battle-tech-firms-cabinet-whatsapp-david-davis