Fair point. However, I think a lot of the folks they’re targeting are not always British citizens exclusively.
For example, going after suspected intelligence operatives or transnational terrorists will be difficult because they have access to foreign addresses and payment methods anyways. Same goes for the folks involved with white-collar or financial crimes with multiple citizenships.
Unless the main objective are folks hiding illegal content in their iCloud Drive (i.e. pirated files, CSAM, WhatsApp backups containing trade secrets). A backdoor can work then, but wouldn’t that encourage folks to use offline methods? At this point, the whack-a-mole thing would just harm the average person.
These crimes has always existed before the concept of encrypted messaging and cloud storage. The UK might as well just ban encryption at this point lol