How about this as a solution instead: Make it in law that if a convicted offender is found to have used a messaging platform to send/receive material, then hea

How about this as a solution instead: Make it in law that if a convicted offender is found to have used a messaging platform to send/receive material, then heavily fine the company that was used in the messaging - but only if they had failed to report the offence. To operate in the UK the company has to pay these fines. This puts the ball in the company's court - like it should be. Now the messaging-platform could claim that since it's E2EE they have no visibility on what's being sent/received, but I believe technical solutions exist for this - E.g. pictures and videos go through a post-processing client-side to remove identifiable data (faces in images, text in images, that sort of thing). The original unmodified image is sent E2EE, and the post-processed one is sent to the company and then it's up to them to identify offending images.