The alternative for Apple is some sort of "malicious compliance", where they offer "Insecure Messages" as a UK-alternative to their Messages app.

It should prominently display an uncloseable banner at the top of the app saying "Due to a surveillance law passed by the Tory government, Apple is not allowed to secure any messages you send or receive in this app".

Users in other countries who communicate with UK users should have a similar but minimisable message, warning them of the same problem.