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.
If companies are required to provide backdoors, presumably what this means in practice is that they release a targeted update to their app with extra code in which breaks the privacy guarantees just for a specific user.
Assuming the app is open source and reproducibly builds, app stores could protect users by implementing Binary Transparency with a public append-only log, but presumably the law will be flexible enough to apply to OS vendors too, forcing them lie about the hashes of the apps that are downloaded.