This is old news at this point, although they will never stop trying.
The United Kingdom’s equivalent of the NSA, the Government Communications Headquarters (GCHQ) wants to put a backdoor in all commercial encrypted chats which will allow law enforcement officers to be inserted into any random conversation of their choice.
Asides from obviously defeating the object of using an encrypted chat in the first place, it would:
End up undermining the security of the system by increasing complexity.
Introduce a mechanism which bad actors could exploit if they were to compromise a central party (like how people keep walking out of the NSA with classified documents on USB sticks).
Such a mechanism could be extended far beyond it’s original purpose like how the ISP filtering scheme in the U.K. is now even policing things like copyright and accidentally clicking on a link to terrorist content can get you severe penalties or result in the police being sent to search you.
A good article from Privacy International which touches upon some of the threats to privacy briefly.